• Custom tabs for grouping (ext+treestyletab:group) are available again on Firefox 150+ which never supports injecting of scripts to addon-hosted contents due to the bug 2011234. See #2356 in Sidebery for more details of affections of the bug.
• Fix wrong order of header and counter items for collapsed native tab groups after native groups are moved. (regression on 4.2.10)
• Stabilize tree restorations based on tree information stored to each tab (by tkng, thanks!)
• Don't leave phantom group and collapsed members counter after a collapsed native group is closed.
• Keep tree structure on detaching all children on some situations (including #3858 by tkng, thanks!)
• Suppress confirmation about closing multiple tabs if the number of closing tabs are smaller than the threashold 25 (same to the default value of browser.sessionstore.max_tabs_undo. If you changed it, please synchronize the threashold to the value of the option) like Firefox does, even if the confirmation is enabled. (by tkng, thanks!)
• Many performance improvements with optimizations and fixed leaks (#3854, #3865, #3857, #3870, #3872 and #3876 by tkng. Thanks!)
• Many cleanup of codes for future maintainability (#3853, #3855, #3856, #3859, #3860, #3862, #3863, #3864 and #3866 by tkng. Thanks!)
• Reduce dead codes around custom event handling and needless exports, for better maintainability.

• Show new tab action selectors on both side of the new tab button correctly, when both container selector and relation selector are active. (by bb010g, thanks!)
• Restore tree of tabs certainly on crash recovery and/or manual restoration of the previous session. (by tkng, thanks!)
• Fix handling of closed tree restoration: multiple tabs of a closed tree are now restored with single action correctly. (by tkng, thanks!)
• Many performance improvements with optimizations (#3831, #3836, #3837, #3838, #3839, #3843, #3847 and #3848 by tkng. Thanks!)
• Some performance improvements with optimizations around reduced reparsing of JSON strings while dragging and reduced reflowing.
• Apply and update auto-sticky state of tabs correctly, even if they are specified by other addons. (by tkng, thanks!)
• Update custom tooltip text more correctly. (by tkng, thanks!)
• Many cleanup of codes for future maintainability. (#3832, #3833, #3835, #3844, #3849, #3851 and #3852 by tkng. Thanks!)
• Suppress errors from handling of successor tabs when a tab is closed before the successor information is updated.

• Fix ability to drop items before the first tab when there is no pinned tab.
• Increase the default delay to show the drop area for tabs to be pinned (500ms to 740ms: you can configure it with a secret option pinInteractionCueDelayMS).
• Prevent grayed favicons of restored tabs in the first window restored from the previous session explicitly.
• Treat discarded tabs as pended correctly, even if restoration from cache is failed.
• Hide "unload tab" context menu command for already discarded tabs, like Firefox does.
• Reduce needless re-rendering of virtually scrolled tabs when pinned tabs are scrolled.
• Update en locale to fix a typo by purplnecropanda. Thanks!

• Add ability to pin/unpin tabs by drag and drop. Now you can drop pinned tabs below/between unpinned tabs, and unpinned tabs above/between pinned tabs. This simulates behaviors of tab dragging on Firefox's vertical tabs.
• Disallow to set the height of the pinned tabs area smaller than the height of a pinned tab.
• Use label text for the "new tab" command in the context menu on tabs same to Firefox's native one, if possible.
• Show dropdown list in options with colors more easy-to-read, on the dark color scheme.
• Expose the expert option to control the position of the context menu popup for native tab groups.
• Update ja locale by tkng. Thanks!

• Support ability to keep the active tab in a collapsed group visible on Firefox 142 and later.
• Prevent to show simulated tab preview panel on non-tab items.
• Prevent "shaking" of pinned tabs when the height of the pinned tabs area is narrower than a tab.
• Restore ability to drop dragged items to the blank area of the tab bar. (regression at 4.2.0)
• API: Fix container assignment for extra tab contents below tabs by emmanuel-ferdman. Thanks!
• Update uk locale by Taron-art. Thanks!

• In-content panels (tab groups menu and tab previews) handle events correctly. (regression at 4.2.3)
• Never mix reactions for custom dialogs even if multiple dialogs are shown at a time.
• Update uk locale by Taron-art. Thanks!

4.2.3 (2025.6.23)

• Context menu commands now works even if it is opened on a blank space (by joanbm. Thanks!)
• Add an expert option to activate/deactivate native tab groups management UI (corresponding browser.tabs.groups.enabled of Firefox itself).
• Scroll pinned tabs container when an out-of-view pinned tab become active.
• Remove dragged tabs from their group when they are dropped before a group header explicitly.
• Don't create needless new group when a last member is dropped before the next another group but the tab is not moved actually.
• Disallow to resize pinned tabs area size larger than required.
• Unblock user operations correctly after drag-and-drop of tabs across windows. (regression at 4.2.0)
• Treat focused or active tab as the context tab if the context menu is opened via the keyboard (ex. Shift-F10 on Windows).
• Accept more cases of dropped native tabs.
• Delete obsolete option to deactivate using of tab multiselection API.

4.2.2 (2025.6.9)

• Pinned tabs are rendered with expected position after rearranging by drag-and-drop. (regression at 4.2.0)
• Apply designed colors always for the "Sidebar" skin, even if the "Dark" color scheme is active. (regression at 4.2.0)

4.2.1 (2025.6.7)

• Scroll to the active tab correctly with disabled animation mode. (regression at 4.2.0)
• Detect sidebar position automatically even if the window is in a screen placed leftside of the primary screen.
• Scroll to the new active tab and keep it in the viewport more correctly even if there are going-to-be-collapsed and/or sticky tabs.
• API: a scroll type message now accepts a new parameter group to scroll the tab bar to the specified tab group.

4.2.0 (2025.6.2):

• Drop support of Firefox older than 139.
• Add support of native tab groups feature of Firefox itself. TST will create a new tab group of Firefox itself instead of modifying tree, when you drag a tab to the head side edge of another tab.
• Show muted state indicator and sharing state indicator (for microphone, camera and screen) at the position same to the one on Firefox's native tab.
• API: Some notification type messages (ex. tab-mousedown) can contain Firefox's tab groups as a tab (tree item). They have properties similar to regular tabs but there is one difference: they have "group" as the value of their type property.