Wikimedia Commons has media related to Commons users who use Mozilla Firefox.

This category is for Wikipedians who use the Mozilla Firefox web browser. Please only specify your user page in this category, and not any sub-pages. You can also add this to put a badge on your userpage:

Code	Result
((User:Technical 13/Userboxes/Browser))	This user contributes using: Mozilla Firefox	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:Technical 13/Userboxes/Browser|FireFox))	This user contributes using: Mozilla Firefox	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:Technical 13/Userboxes/Browser|ff|ESR 17.0.4))	This user contributes using: Mozilla Firefox ESR 17.0.4	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:Technical 13/Userboxes/Browser|FF|20.0.1))	This user contributes using: Mozilla Firefox 20.0.1	Usage • Category:Wikipedians who use Mozilla Firefox •
((Template:User Firefox))	This user contributes using Firefox.	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:Mkdw/Mozilla Firefox))	This user contributes using Mozilla Firefox.	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:The Raven's Apprentice/Userboxes/User Firefox))	This user contributes using Firefox.	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:Jruderman/Userboxes/FirefoxTrunk))	This user contributes using Firefox Nightly.	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:KyraVixen/Userboxes/browser:PrefersFirefox))	This user prefers Mozilla Firefox.	Usage • Category:Wikipedians who use Mozilla Firefox •
((User:Realmdee/FirefoxESR))	This user contributes using Firefox ESR.	Usage • Category:Wikipedians who use Mozilla Firefox •

If you don't like any of the available userboxes above or are just anti-userbox, you may just add yourself to this category with:

[[Category:Wikipedians who use Mozilla Firefox|((SUBST:#titleparts:((SUBST:BASEPAGENAME))|1))]]