Can't read text in web forms since firefox uses dark theme by default [duplicate]
Solution 1:
This is a known issue. You can find bug reports here:
Bug 70315 - text in menus and boxes unreadable if using dark GTK theme
Bug 519763 - dark linux gtk theme makes pages look bad; "use system colors" should apply to input elements as well as fonts, or separate option needed
The suggested work around is to create a userContent.css file in your firefox profile as per these instructions from the Arch Wiki.
Solution 2:
One solution is to edit the userContent.css for firefox. I had this issue previously with the Ubuntu Software center and did something similar.
You can find a related post about it here: http://forums.fedoraforum.org/showpost.php?p=1703251&postcount=3
I am not sure if this fix still works since the 16.04 update, but it should be applicable.