I've just upgraded ~10 Fedora v34 -> v35 boxes.

DE on all is KDE. all X11; wayland's disabled in sddm.conf

On v34, Firefox 93 -- whether installed from upstream tarball or F34 distro-RPMs -- worked great.

On v35, menu-bar items (File/Edit/View/History/etc) are non-functional.
Nothing on hover. Clicking on them locks up FF, requiring a kill -9 to exit.

FF's Settings with pull-downs, e.g. 'Default Font' drop down, also are non-responsive.
Unlike the MenuBar items, the Settings' clicks do NOT lock it up.

Everything else non-drop-down related in FF appears to function OK.

Exactly the same MenuBar item issues in TBird.

This behavior is reproducible whether using upstream or distro installs, of either TBird or FF.

The problem is insensitive to any/all of,

-- mozilla & KDE cache clearing
-- clean FF/TBird installh empty profiles
-- extensions enabled or disabled
-- hardware acceleration enabled or disabled

, and behaves as reported in each case.

Afaict, no other apps on my system have any such issues.

I've specifically isntalled & tested Midori & GNOME Web (epiphany), both GTK apps.
Simlarly, browser Chrome variants -- Brave/Vivaldi/Chrome/Chromium -- all behave, too.

On click&freeze of any MenuBar item there's no output in journal, and nothing output to browser console.

So far, no other oddities I've found on the upgraded, f35/KDE installs