I forget what specific site it was, but I recently experienced this with some website a doctor's office uses for online appointments. The site straight up said that it didn't work with Firefox and to open it in a different browser. I ended up having to use Edge because I uninstalled Chrome as soon as I had switched back to Firefox.

My local power company's site is broken in strange ways in Firefox. Bars for recent bill amounts are all the same length, usage page throws an error and won't open, a link in the dropdown for a home energy analyzer is completely missing.