Well, my troubleshooting would have revolved around reviewing network activity, but that wouldn't seem to be a productive avenue of enquiry if Chrome can access what Firefox can't - I would suggest totally un-installing Firefox, and then performing a fresh install
(if possible, try to live without whatever 'customisations/bookmarks' have been set up since you last installed it, as the 'bug' could lie within them, and restoring the settings in a fresh install might perpetuate the 'bug').