I’ve this problem too, but not only with firefox, but with all browser that I’ve: Firefox, Chromium, Opera.
All plugins are deactivated.
This problem suddenly appear after I changed my permalink structure (and removed .htacces file), and now on my workstation won’t go away, even if i restored the .htacces file and permalink structure is the default one again.
The strange thing is that I’ve always worked on child theme, and even if I try to use the parent theme this problem is still appear (yes, i flushed cache browser)
On my laptop I’ve 1:1 copy of the website installed (file and database), and no problem in there.
If I try from my workstation to connect to laptop i see everything fine, if from my laptop connect to workstation icons doesn’t work.
here is a screenshoot, any help is appreciatied!
I apologize for my bad English,
regards
Dario