Framework note: This essay applies the carry-through criterion to software configuration. The question is what essential value-add could not be achieved through existing tools. Command palette — Vimium already exists. Vertical tab sidebar — Sideberry already exists. Tiling and split views — most operating systems and window managers already handle this well. The only thing missing was the CSS that makes these tools feel like a single, cohesive browser. That is what Fennec is — and nothing more. See What Carries Through for the structural claim.
This essay discusses 🦊 Fennec, a project I built — a single CSS file that makes Firefox usable with vertical tabs and a zen mode toggle, and nothing else.
The Single-File Principle
I have been taking some inspiration from Jeremy Howard’s FastHTML. It does not require a frontend build step: a small app can live in a single Python file you can read top to bottom. Auth, routes, database, templates — no bundler, no deployment pipeline, no ops. You look at the file. You understand it. You change it. You ship it.
This is not a limitation. It is a design decision. The single-file constraint is load-bearing. It means the application is legible to its author six months from now. It means there is no dependency graph that rots while you are not looking. It means the maintenance cost asymptotically approaches zero rather than compounding toward infinity.
Fennec applies this principle to browser configuration.
The Landscape
There are three approaches to making Firefox usable in 2026.
Zen Browser
Zen is a full fork of Firefox with vertical tabs, workspaces, split views, theming, a command palette, glance previews, and a mod store. It has 40,000 stars on GitHub, an aggressive release cadence, and a community that genuinely cares about the product. Zen is good software. It has also been working through real challenges — performance regressions that required removing features like rounded corners on Linux, and DRM content issues with Widevine licensing that have made streaming services unreliable. These are not criticisms. They are the inherent costs of maintaining a fork. Every fork accumulates the surface area of its own ambition, and Zen is ambitious.
FF-Ultima
FF-Ultima is the flagship userChrome.css modification — a theme with tab size options, autohiding options, spacing options, accessibility options, color scheme options, Sidebery integration options, Auto Tab Color integration options, and a settings surface in about:config that rivals some applications. Its own tagline is “the Firefox theme with too many options.” FF-Ultima is honest about what it is. For users who want deep control over every visual detail, that breadth of configurability is genuinely valuable. It is a configuration system for people who want to configure.
Fennec
Figure 1: Fennec CSS with sidebar shown
Full UI mode. Sideberry’s sidebar holds the vertical tabs and the URL bar. Everything Firefox normally scatters across the top of the window lives in one panel that you can toggle away.
Figure 2: Fennec CSS with sidebar hidden
Zen mode. The sidebar is gone. The content occupies every pixel. The window stays tiled. This is the state stock Firefox cannot reach without going fullscreen.
What Fennec Is
Fennec is a userChrome.css file that moves the URL bar into Sideberry’s sidebar, hides the rest of the chrome, and provides a toggle that switches between full UI and zen mode. There is a companion CSS file for Sideberry’s style editor. That is the entire project.
There is no settings surface. There is no theme store. There are no layout modes. There is no command palette. There is no about:config integration. There is no autohide system. There is no installer script.
You copy a CSS file into your profile’s chrome folder. You paste another CSS file into Sideberry’s style editor. You restart Firefox. You are done.
There is no user.js. This is not a trivial omission. Many userChrome themes ship a user.js that modifies Firefox’s internal about:config preferences — flags that control security behavior, TLS settings, certificate handling, safe browsing, and update mechanisms. The user.js is typically deleted after first launch, but the settings it wrote persist silently in prefs.js. You are trusting someone else’s about:config modifications without necessarily understanding each flag, and you are trusting that none of them weaken Firefox’s security model in ways you did not intend.
Fennec accomplishes with CSS alone what most other projects have required user.js modifications to achieve — sidebar toggling, UI reordering, the URL bar relocation into Sideberry’s panel. This was a deliberate design decision. Solving these problems in pure CSS is harder upfront. It requires more careful planning, more knowledge of Firefox’s internal DOM structure, and more willingness to find creative solutions within the constraint. But the constraint is the point. CSS can change how Firefox looks. It cannot change how Firefox behaves. This helps preserve the security boundary between presentation and configuration. Fennec does not require changing Firefox’s internal preference state, which avoids expanding the trust surface in the way user.js-based setups can.
The Tiling Problem
The zen mode toggle is not a convenience feature. It solves a structural gap in Firefox.
The only built-in way to hide all of Firefox’s chrome — the tab bar, the URL bar, the navigation buttons — is fullscreen mode. Fullscreen mode is mutually exclusive with tiling. If you run a tiling window manager, or if you simply want your browser occupying half the screen with a terminal next to it, Firefox gives you no option. You see the chrome, or you go fullscreen and lose the ability to tile. There is no middle state.
Fennec’s zen mode is that middle state. The UI disappears, the window remains tiled, and the content occupies every pixel of the window’s allocated space. Toggle Sideberry off and the browser becomes a content pane. Toggle it back on and the tabs and URL bar return. The window never changes size, never leaves the tiling layout, never disrupts whatever is running next to it.
Why Complexity Is the Failure Mode
The failure mode of browser customization is not the initial setup. The initial setup is fun. You find the perfect CSS, you tune it, you screenshot it, you post it to r/unixporn. The failure mode is month four, when Firefox updates and something breaks, and you open a directory full of CSS imports that reference about:config flags you do not remember setting, and you realize the cost of your configuration exceeds the cost of the problem it solved. You abandon it. You go back to stock Firefox. You feel vaguely defeated.
FF-Ultima has a wiki. It has a gituserChrome automation tool. It has video installation tutorials. It has a settings surface with dozens of configurable flags.
(FF-Ultima’s getting started guide gives a sense of the configuration surface.)
A project with a broad configuration surface naturally develops documentation and tooling around it. The support infrastructure is well-built — the maintainer clearly cares about the experience — but its existence indicates a different maintenance model than what Fennec aims for.
Zen is a fork. A fork means you are running someone else’s build of Firefox. You are downstream of both Mozilla’s decisions and Zen’s decisions. When Zen updates multiple times per week, you update multiple times per week. When Zen makes a UI choice you disagree with, you file an issue or you live with it. When performance requires removing visual features to stay usable, you lose them whether you had the problem or not. When Mozilla ships a breaking change, you wait for Zen to absorb it. The fork is a dependency you do not control.
But the deeper question is what Zen is building toward. There is real value in having a command palette, vertical tabs, and split views integrated into a single coherent interface — the experience can be more polished than bolting together separate tools. But in my view, many of these capabilities are already handled well by mature standalone tools: Vimium for command palettes, Sideberry for vertical tabs, a tiling window manager for split views. Each reimplementation inside the fork is a new surface that must be maintained, debugged, and kept compatible with upstream Firefox. The maintenance cost of a fork is proportional to the distance between the fork and upstream. Zen accepts that distance as the cost of integration. Fennec minimizes it by building only the glue.
Fennec has none of these problems because Fennec has almost nothing. You are running Firefox. You are running Sideberry, a mature and well-maintained extension. On top of that sits a CSS file you can read in five minutes. When Firefox updates, the CSS either still works or it does not, and if it does not, you fix it in the same file you already understand because you have read the whole thing. The recovery path from any breakage is: open one file, find the broken selector, fix it.
In four months of daily use across Firefox updates, Fennec has required exactly one patch — a single CSS fix.
That is the maintenance cost of a project with almost no surface area. No dependency chain. No upstream maintainer whose schedule you are beholden to. No settings state to debug.
The Carry-Through Test
The question for any tool is not “does it work today?” The question is “will I still be running it in two years, and at what cost?”
A Zen installation carries through only if Zen does. An FF-Ultima installation carries through only if you are willing to maintain a relationship with its configuration surface indefinitely. Both are conditions external to you or costs that compound over time.
A Fennec installation carries through because there is almost nothing to maintain. One CSS file. One extension. Firefox. The surface area of possible breakage is small. The surface area of your own comprehension covers the entire project. There is nothing you do not understand about your own browser.
That is the value proposition. Not features. Not options. Not configuration depth. Comprehension. The entire system fits in your head, which means you own it in a way that no amount of features can substitute for.
What Fennec Is Not
Fennec is not for everyone. If you want workspaces, a command palette, and split views built into your browser, Zen is a strong option — the integrated experience is its whole point. If you want a settings surface that lets you tune every pixel, use FF-Ultima. Both are good projects solving real problems for people who have those problems. I prefer to leave tiling to the window manager and keep the browser thinner. That preference is what Fennec is built around.
Fennec is for people who want vertical tabs, a clean toggle between UI and zen mode, and a keyboard-driven workflow — and who want those things delivered by the tools that already do them best, held together by the thinnest possible integration layer. Sideberry, Vimium (optionally, to complement the keyboard-driven flow), and a CSS file. That is the browser. There is nothing else to manage, nothing else to break, and nothing else to understand.
The smallest thing that works is not the worst version. It is the version that carries through.