STRs:

• enable the tab panel UI: set browser.ctrlTab.recentlyUsedOrder to true
• press ctrl+shift+tab

Expected result: the tab panel UI should open
Actual result: nothing happens

I recently had browser.ctrlTab.recentlyUsedOrder turned on in my regular profile (not sure how), and I was surprised that ctrl+shift+tab did nothing.

Once you open the tab panel via ctrl+tab, ctrl-shift-tab will cycle in the opposite order. But it won't start the UI. I am not sure if this is expected or if it is bug?

In addition as suggested in bug 1631843 and as a system Alt+Shift+Tab behaviour parity (at least on Windows platform) Ctrl+Shift+Tab should probably select the least recent tab.

What leads to interesting observation seen in current Firefox, where Ctrl+Shift+Tab activates the "⌵ List all tabs" button, which appears when tab bar is overflowed. This leads to super confusing experience with following STR:

• have tab bar overflown,
• pres Ctrl+Shift+Tab and hold Ctrl ("All tabs" list opened),
• pres Ctrl+Tab, i.e. release Shift and pres Tab again.

Such sequence normally returns focus to the current item in OS Alt+Tab counterpart.

In current Firefox (75+, Windows) it opens 'regular' "CtrlTab modal Panel" above All tabs list.

Moreover, further presses on Tab key with Ctrl still pressed results in switching focus inside CtrlTab modal panel between first item and newly introduced "List all %count% tabs", without mean to focus other items in this "CtrlTab session", effectively jeopardizing it.

And last argument to add -- since I've observed that parity with other more mainstream browsers currently plays very important role in Firefox development -- there is probably not a single browser or even application with tabbed interface (what is something Firefox or its predecessors popularized) which uses Ctrl+Tab to switch to "next item" (whatever paradigm that "next" means, be it temporal or spatial proximity) but does not use Ctrl+Shift+Tab for similar action in reversed order. Not to mention that browser CtrlTab panel mimicking OS AltTab panel is not established pattern in browser landscape, so introduction of it into stable branch in such sad condition seems a bit weird.

Considering that browser.ctrlTab.recentlyUsedOrder now defaults to true for new profiles, it makes the very first minutes of using freshly installed Firefox really, really painful experience I don't rather want not have to excuse to my friends and coworkers.

Sorry for rant. For not to be so snarky, I highly appreciate that we still have reliable Ctrl + Page Up / Ctrl + Page Down keyboard shortcuts for 'sane' tab switching and i quite understand it is a good thing to explore and introduce new and perhaps better features alongside those old and proven ones. Thanks for all your efforts.

The severity field for this bug is relatively low, S3. However, the bug has 3 duplicates.
:dao, could you consider increasing the bug severity?

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Fixed by bug 1786011.