For what it's worth, there is a very clean workaround in the ArchWiki:
https://wiki.archlinux.org/index.php/Firefox#Unreadable_input_fields_with_dark_GTK.2B_themes
I had seen some other userContent.css "fixes" that didn't get everything or overrode too much, breaking legit stylesheets, but this seems to only override the default and shouldn't override any actual website css.
For what it's worth, there is a very clean workaround in the ArchWiki:
https:/ /wiki.archlinux .org/index. php/Firefox# Unreadable_ input_fields_ with_dark_ GTK.2B_ themes
I had seen some other userContent.css "fixes" that didn't get everything or overrode too much, breaking legit stylesheets, but this seems to only override the default and shouldn't override any actual website css.