Mozilla to Kill Classic Firefox Sidebar July 21, Forcing New Interface on All Users
Mozilla will permanently retire its classic Firefox sidebar on July 21 when it ships version 153, replacing the long-standing interface with a redesigned vertical pane that removes several features power users rely on daily.
The company will deprecate the legacy UI flag entirely in the release, locking all users into the new sidebar with no opt-out path.
What Changes — and What Breaks
The classic sidebar opened as a minimal, dropdown-driven panel letting users switch between bookmarks and history with a single click at the top of the pane.
The new version replaces that dropdown with a wider, icon-heavy vertical strip that consumes more horizontal screen space — a meaningful trade-off on smaller or lower-resolution displays.
The redesign also breaks at least two established workflows that power users depend on.
Dragging a history entry into the tab bar to open it in the background no longer works. Selecting multiple non-consecutive links using Ctrl and the spacebar to batch-open tabs is also broken — the new interface hijacks focus and loads the page the moment a user hits the spacebar.
Discussion threads on r/firefox show users flagging the breakage after receiving the update, with many describing the loss of those workflows as a significant regression.
No Escape Route Exists
Users who prefer Firefox’s vertical tabs feature face a particularly hard constraint.
Navigating to Settings > General > Browser Layout and toggling off “Show sidebar” forces the browser back to a horizontal tabs layout. Switching vertical tabs back on immediately resurrects the new sidebar alongside it.
The two features are hardwired together at the implementation level, leaving no configuration that preserves vertical tabs while reverting to the classic sidebar experience.
That means Mozilla is presenting users with a binary choice: accept the new interface in full, or abandon vertical tabs to approximate the old layout.
Mozilla’s Response
Support staff on Mozilla Connect confirmed the third-quarter removal date and acknowledged known regressions, including a broken multi-delete function for history entries.
Patches for those regressions are in the works, staff said, but have not shipped yet.
Until fixes arrive, users must manually right-click individual links to delete them from history — a workaround that replaces what was previously a multi-select batch operation.
Mozilla has not published a public timeline for when the regression patches will reach stable builds.
Background
Firefox holds roughly 2.6% of the global desktop browser market, according to StatCounter data from early 2025.
The company has pushed several interface overhauls in recent years as It Works to modernize Firefox against competition from Chrome and Edge, both of which have introduced their own vertical tab implementations.
Firefox 153 is scheduled for release July 21 as part of Mozilla’s standard four-week release cycle.
