Another upgrade, another regression (don't want firefox icon on new tabs)
Same question as I had a few months ago:
The solution given worked fine... until I just upgraded from 68.0.2 to 69.0.3 (and now it no longer works).
Additional System Details
- User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0
Firefox no longer loads userChrome.css or userContent.css by default improving start-up performance. Users who wish to customize Firefox by using these files can set the toolkit.legacyUserProfileCustomizations.stylesheets preference to true to restore this ability.
See the Fx 69 release notes.