Firefox 136 shipped vertical tabs, which was the right foundation — but the sidebar is still "the tab strip, rotated." Arc treated it as the browser's primary navigation surface, and with Arc now in maintenance mode there's a large, technically literate user base looking for a new home. Four things stand between Firefox and being the obvious destination:
- Pins should remember a URL. In Arc, a pinned tab is bound to a home address — navigate away and one click (or a restart) returns it there. Firefox pins are just small tabs that drift, so they stop being reliable navigation targets. Store a home URL per pin, restore to it on restart, add "reset to pinned URL."
- A favorites tier above pins. Arc has three tiers with three different lifetimes: favorites (always visible, icon-only, global), pinned tabs (per-space, named, ordered), ordinary tabs (transient). That tiering is what lets a sidebar hold 60 tabs and stay usable. Firefox has two, weakly separated.
- Persistent folders for pinned tabs. Vertical space is cheap and supports hierarchy — that's the actual argument for vertical tabs over horizontal. Collapsible, drag-and-drop folders whose state survives restart.
- A sidebar that hides to zero width. Arc's hidden state reserves no space at all; hovering the window edge slides it back as an overlay above the page, without reflowing content. Firefox's collapsed state keeps an icon rail and pushes content when it expands. This sounds cosmetic but it's the mechanism that makes a rich sidebar affordable on a 13" screen. (Configurable delay + click-instead-of-hover option for accessibility.)
All of it is additive and can sit behind the existing vertical-tabs prefs with today's behaviour as default. Containers already map cleanly onto Arc's Spaces — the harder primitive is arguably built. Suggested order by value-to-effort: URL-bound pins → true hide/hover reveal → folders → favorites.
