All the icons got all big and wonky. I even had trouble making this post.
We did update Lemmy to the recent version, which might explain the timing. Only some users seem to be getting it. There weren't any big changes with this version
Can you try starting Firefox in troubleshoot mode?
https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems