Yeah, I think that one of your extensions is messing with the forum's CSS somehow.
I may have isolated the Ghostery extension as one cause.
I created a completely new Firefox profile and re-installed each extension, testing several websites after each one.
Everything was fine until I installed Ghostery, when the images behind this forum's navigation tabs disappeared.
This happened even before I enabled Ghostery's blocking (by default it should block nothing), just installing is enough to mess up the page.
A pity, as I really liked Ghostery's blocking of all the tracking and spyware that's on almost every webpage these days.
I also discovered several extensions have no clean uninstall in Firefox and they leave a mess of configuration settings behind.
Thanks for all the feedback though, I'll keep this Ghostery-less profile only for trusted websites and continue testing in another profile.