I have this behavior on firefox 94 from sid, I suspect the MOZ_APP_LAUNCHER environment variable, which is set by thunderbird and evaluated in browser/components/shell/nsGNOMEShellService.cpp:135
(Filtered) environment of firefox, when started through thunderbird (this wants thunderbird.desktop as default browser): We have thunderbird in $MOZ_APP_LAUNCHER and firefox in $GIO_LAUNCHED_DESKTOP_FILE DESKTOP_SESSION=mate GDM_LANG=de_DE.utf8 GIO_LAUNCHED_DESKTOP_FILE=/usr/share/applications/firefox.desktop GIO_LAUNCHED_DESKTOP_FILE_PID=3669595 HOME=/home/j LANG=C.UTF-8 LC_MEASUREMENT=en_DK.UTF-8 LC_PAPER=en_DK.UTF-8 LC_TIME=en_DK.UTF-8 LD_LIBRARY_PATH=/usr/lib/firefox LD_PRELOAD=libmozsandbox.so LOGNAME=j MATE_DESKTOP_SESSION_ID=this-is-deprecated MOZ_APP_LAUNCHER=/usr/bin/thunderbird MOZ_APP_SILENT_START= MOZ_ASSUME_USER_NS=1 MOZ_CRASHREPORTER_[...] MOZ_LAUNCHED_CHILD= MOZ_PROFILER_STARTUP= MOZ_SANDBOXED=1 MOZ_SANDBOX_USE_CHROOT=1402653184 TB_FAIL=0 TB_HELP=0 TB_VERBOSE=0 XDG_CURRENT_DESKTOP=MATE XDG_[...] XDG_SESSION_TYPE=x11 (Filtered) environment when started through MATE-Panel: (this wants firefox.desktop as default browser) GIO_LAUNCHED_DESKTOP_FILE=/usr/share/applications/firefox.desktop GIO_LAUNCHED_DESKTOP_FILE_PID=3674365 LD_LIBRARY_PATH=/usr/lib/firefox LD_PRELOAD=libmozsandbox.so MOZ_APP_SILENT_START= MOZ_ASSUME_USER_NS=1 MOZ_CRASHREPORTER_[...] MOZ_HEADLESS=1 MOZ_LAUNCHED_CHILD= MOZ_PROFILER_STARTUP= MOZ_SANDBOXED=1 MOZ_SANDBOX_USE_CHROOT=11476395008 PATH=/home/j/.cargo/bin:/home/j/config/bin:/home/j/.local/bin:/home/j/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/snap/bin:/sbin:/usr/sbin PWD=/home/j XDG_SESSION_DESKTOP=mate XDG_SESSION_TYPE=x11
