Chrome version 154.0.8037.57-1 updated its statically linked libfontconfig, and now its cache is stomping on the user-level font cache.
To fix this, press Ctrl + Alt + t to open a terminal, and run
rm -rf ~/.cache/fontconfig
And then reboot.
Note that opening Chrome (or a Chromium-based program) will break it again. I edited my ~/.local/share/applications/google-chrome-beta.desktop to do this:
Exec=env XDG_CACHE_HOME=/home/user/.cache/chromecache/ /usr/bin/google-chrome-beta %U
More info:
https://issues.chromium.org/issues/565052667
https://gitlab.freedesktop.org/fontconfig/fontconfig/-/work_items/538
You can make changes to the Firefox UI with CSS coding yourself: https://www.userchrome.org/ For example, my Firefox looks currently like this (looks pretty much the same in a non-private session too), just like my operating system with KDE:
It’s a complex mix of addons, custom CSS, custom JavaScrip, some custom settings in Firefox and so on. You can copy CSS codes into your userChrome.css file, some good sources I use are: