Hi,
as far as I can make out, Facebox should work as is in Firefox.
I found this site:-
http://admins.guildportal.com/Guild.aspx?GuildID=3780&TabID=27537&ForumID=22042&TopicID=7136500&Page=1#35701592
and there is a section that says:-
Images
You can have the popup box show your images too! All you need to do is add "rel=facebook" to any link (in red)
<a rel="facebox" href="http://i201.photobucket.com/albums/aa236/Mottie1/SMRT.jpg">Kitty!</a>
Test it out the unmodified window here: Kitty!
I clicked the link ‘Kitty!’ and the modal window appeared for me, I am currently using FF5.
Prehaps you could ask there.
Bear in mind that Facebox is now no longer updated by the developer.
Regards…,
MT