Why the author left Zen
Singh describes Zen as a Firefox fork that puts more thought into tab management and the interface while keeping the Gecko engine. He liked it because it felt close to Arc, which he calls "RIP", and it let him stay off Chromium. Zen's workspaces were a big part of the appeal. After trying many forks, he decided the features that drew him to them could now be had in stock Firefox.
His claim is narrow. He says he doesn't need Zen just because he prefers its layout, and that regular Firefox covers enough of his needs. The piece offers no benchmarks, no controlled comparison, and no evidence of any gain in speed, privacy or security. Treat it as a workflow preference.
Tier one: native vertical tabs and tab groups
Start here, because it needs no extension. Mozilla's own staff post on Mozilla Connect says the updated sidebar and vertical tabs rolled out to all release users on Firefox 136. Wait, that post's date reads as 10 March 2025, and Mozilla's announcement describes the route this way: in Settings, General, Browser Layout, tick Show sidebar. You can also right-click the toolbar or tab strip and choose Turn on Vertical Tabs.
The Mozilla blog gives the reasoning behind the feature. It says horizontal tabs work fine until you have more than 10 open. Past that point, titles shrink to nothing. In the author's example, favicons can't tell five GitHub repositories apart, but a vertical list shows more of each title. You can collapse the sidebar when you want the page space back.
Tab groups are the other half. Firefox's own feature page says groups are built in, so you don't need an extension to get started. It also says they keep working in the sidebar, so the layout stays consistent whether tabs sit at the top or the side. The feature page lets you:
- Keep separate sets of tabs for different projects.
- Collapse groups you aren't using.
- Add colors and labels.
The right-click menu on a tab has Add Tab to New Group and Add Tab to Group, and a group's label collapses or expands it. Dragging one tab onto another also makes a group.
Caveat: groups are not Zen-style workspaces. They organize tabs inside one window and one tab list. If you rely on fully separate workspaces with their own contexts, treat this tier as a partial substitute. Don't read it as a clone.
Tier two: Sidebery for panels, trees and snapshots
Sidebery was a popular answer to this problem long before Mozilla shipped native vertical tabs. gHacks noted that Firefox users have relied on extensions like Tree Style Tab and Sidebery for a similar experience. The author uses it for three things:
- Named panels: for example work, personal and research, each with its own tabs.
- Tree-style nesting: pages opened from a parent page sit beneath it and collapse as a branch.
- Snapshots: a saved record of open tabs and windows, so you don't have to bookmark everything one by one.
Hierarchy is the real gap in the native feature. gHacks' early look at vertical tabs observed that the tabs had no hierarchy and all appeared on one level. If your research habit is "open ten links from one page", trees are the reason to install an extension.
To set it up, install Sidebery from Firefox Add-ons, open it in the sidebar and create panels in its settings. Leave the advanced controls for later.
Limits to know about (from Sidebery's Add-ons listing, as relayed in the research notes):
- Firefox doesn't let extensions hide the native horizontal tab bar. Hiding it takes userChrome.css tweaks.
- Some features need optional permissions. These include hiding tabs in inactive panels or folded branches, bookmark or history access, and snapshot auto-export. Check what Firefox asks before you accept.
- Sidebery stores data locally by default. Sync is optional.
- The developer warns of possible conflicts with other extensions that manage tab position or sorting.
Tier three: appearance, toolbar and userChrome.css
The low-risk step is Customize Toolbar. Right-click a blank area of the toolbar or tab strip, then drag controls in or out. Add a theme to match colors. Mozilla's documentation warns that Restore Defaults resets the default theme and reverts vertical tabs to horizontal. Don't click it casually.
The deeper step is userChrome.css, which can change spacing, visibility and placement of browser parts. Singh points to FF Ultima, a community theme. Its GitHub page describes it as a configurable theme with several layouts, including vertical tabs and Sidebery. It says it tries to support extensions like Sidebery automatically.
I'd rather not repeat that sentence's phrasing as a guarantee. The project says it "trys to support" Sidebery, which is a best-effort claim. The repo lists installation steps involving profile files, with a setup wizard folder and documentation.
Risk: Mozilla's support documentation says userChrome.css and about:config changes aren't officially supported and are meant for developers. They can break Firefox or stop working after an update. Mozilla's recovery advice is to set toolkit.legacyUserProfileCustomizations.stylesheets to false and remove or rename the stylesheet in the profile's chrome folder. Keep that handy before you theme anything.
Which route fits you?
| Goal | Best starting point | Upkeep |
|---|---|---|
| Readable titles, less tab squeeze | Native vertical tabs | None |
| Named, collapsible collections | Native tab groups | None |
| Nested research trees, panels, snapshots | Sidebery | Extension updates, permissions |
| Zen-like look and hidden tab bar | FF Ultima plus userChrome.css | Manual, may break on updates |
Reading the story critically
- It's a personal verdict. "Enough" for one person's work and personal tabs may not be enough for you.
- Forks differ. Zen may bundle polish, defaults and workspaces that a hand-built setup only approximates.
- Maintenance moves to you. Forks give you a ready configuration. A stock install plus extension plus CSS makes you the integrator. For a Windows admin, that also means more to document if you support colleagues.
- Native features have been maturing. Mozilla's own staff said at launch they were working on more improvements, including expand-on-hover for vertical tabs. Check your current Firefox version for what has landed.
Bottom line
Try native vertical tabs and groups first, since they're free and supported. Add Sidebery if you want trees and panels. Only go to userChrome.css if you really care about the look, and accept that you may have to fix it after a Firefox update.