Hi Begijn, this could happen if you set up a userChrome.css file. Some of the older rules do not work correctly in Firefox 108.
If you remember the source where you found your rules, I suggest going back and updating from the source. If you can't track that down, try the wizards on r/FirefoxCSS who have been addressing these issues since Firefox 108 came out:
https://www.reddit.com/r/FirefoxCSS/
