Firefox is partly broke since I updated to F43 (initial F43 version was 144.0-3): it always crashes when I try to save a PDF (both with "ctrl+s" and "file">"save page"), or when I click any button that tries to upload files. So it seems opening the file manager is the constant among both, but that is speculation. Examples include the "Durchsuchen" button of this page: https://www.elster.de/eportal/login/softpse , but also the upload buttons for pictures (e.g., profile picture of the speaker) in the talk applications of FOSDEM.

My feeling is that I also have noteworthy performance issues since upgrading to F43. Sine I do not know how far Firefox and Thunderbird are overlapping in their use of other functions/tools/libraries, it might be noted that I already have issues with thunderbird for some time, though that was already before upgrading to F43 while they got worse since the upgrade -> BZ2403355

I just provoked the issue of Firefox in the mentioned page with the "Durchsuchen" button while Firefox was opened through the terminal, thus causing output:

py0xc3@fedora:~$ firefox 
[Parent 43770, Main Thread] WARNING: Could not load a pixbuf from icon theme.
This may indicate that pixbuf loaders or the mime database could not be found.: 'glib warning', file /builddir/build/BUILD/firefox-144.0.2-build/firefox-144.0.2/toolkit/xre/nsSigHandlers.cpp:201

(org.mozilla.firefox:43770): Gtk-WARNING **: 19:30:00.192: Could not load a pixbuf from icon theme.
This may indicate that pixbuf loaders or the mime database could not be found.
**
Gtk:ERROR:../gtk/gtkiconhelper.c:495:ensure_surface_for_gicon: assertion failed (error == NULL): Failed to load /usr/share/icons/breeze/status/16@2x/image-missing.svg: Loader process exited early with status '1'Command:
 "bwrap" "--unshare-all" "--die-with-parent" "--chdir" "/" "--ro-bind" "/usr" "/usr" "--dev" "/dev" "--ro-bind-try" "/etc/ld.so.cache" "/etc/ld.so.cache" "--ro-bind-try" "/nix/store" "/nix/store" "--tmpfs" "/tmp-home" "--tmpfs" "/tmp-run" "--clearenv" "--setenv" "HOME" "/tmp-home" "--setenv" "XDG_RUNTIME_DIR" "/tmp-run" "--setenv" "XDG_RUNTIME_DIR" "/run/user/1000" "--symlink" "/usr/lib" "/lib" "--symlink" "/usr/lib64" "/lib64" "--ro-bind-try" "/etc/fonts/conf.d" "/etc/fonts/conf.d" "--ro-bind-try" "/etc/fonts/fonts.conf" "/etc/fonts/fonts.conf" "--ro-bind-try" "/home/py0xc3/.cache/fontconfig" "/home/py0xc3/.cache/fontconfig" "--ro-bind-try" "/usr/lib/fontconfig/cache" "/usr/lib/fontconfig/cache" "--bind-try" "/home/py0xc3/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "/home/py0xc3/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "--setenv" "XDG_CACHE_HOME" "/home/py0xc3/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "--seccomp" "302" "/usr/libexec/glycin-loaders/2+/glycin-svg" "--dbus-fd" "295" (gdk-pixbuf-error-quark, 0)
Bail out! Gtk:ERROR:../gtk/gtkiconhelper.c:495:ensure_surface_for_gicon: assertion failed (error == NULL): Failed to load /usr/share/icons/breeze/status/16@2x/image-missing.svg: Loader process exited early with status '1'Command:  "bwrap" "--unshare-all" "--die-with-parent" "--chdir" "/" "--ro-bind" "/usr" "/usr" "--dev" "/dev" "--ro-bind-try" "/etc/ld.so.cache" "/etc/ld.so.cache" "--ro-bind-try" "/nix/store" "/nix/store" "--tmpfs" "/tmp-home" "--tmpfs" "/tmp-run" "--clearenv" "--setenv" "HOME" "/tmp-home" "--setenv" "XDG_RUNTIME_DIR" "/tmp-run" "--setenv" "XDG_RUNTIME_DIR" "/run/user/1000" "--symlink" "/usr/lib" "/lib" "--symlink" "/usr/lib64" "/lib64" "--ro-bind-try" "/etc/fonts/conf.d" "/etc/fonts/conf.d" "--ro-bind-try" "/etc/fonts/fonts.conf" "/etc/fonts/fonts.conf" "--ro-bind-try" "/home/py0xc3/.cache/fontconfig" "/home/py0xc3/.cache/fontconfig" "--ro-bind-try" "/usr/lib/fontconfig/cache" "/usr/lib/fontconfig/cache" "--bind-try" "/home/py0xc3/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "/home/py0xc3/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "--setenv" "XDG_CACHE_HOME" "/home/py0xc3/.cache/glycin/usr/libexec/glycin-loaders/2+/glycin-svg" "--seccomp" "302" "/usr/libexec/glycin-loaders/2+/glycin-svg" "--dbus-fd" "295" (gdk-pixbuf-error-quark, 0)
Redirecting call to abort() to mozalloc_abort

ExceptionHandler::GenerateDump attempting to generate:/home/py0xc3/.mozilla/firefox/jcm6gadg.default-release/minidumps/0eb5d900-cb62-39da-283a-3b264ad9c569.dmp
ExceptionHandler::GenerateDump cloned child 44602
ExceptionHandler::WaitForContinueSignal waiting for continue signal...
ExceptionHandler::SendContinueSignalToChild sent continue signal to child
ExceptionHandler::GenerateDump minidump generation succeeded
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.

Sorry for not providing a more appropriate bug report, but the time I can spare at the moment is limited.

Supplement to clarify: the performance issues are limited to Mozilla Firefox and Thunderbird: the performance issues are not a big thing in Firefox (more a subjective perception), while Thunderbird along with BZ#2403355 is more or less broke as everything takes several seconds up to a minute depending on the operation/click (I'm not convinced that the two issues are related, but it might be mentioned). So it ain't performance issues on the system in general or so.