Googling turns up a few issues with vBulletin and FF over the years over this issue.... not sure who is to blame, as that depends on lots of variables. I don't think it's specifically a vBulletin/DRC issue, though, as I always browse the site in Opera - both at home and at work - and have never seen this issue before. So most likely, it's something that vBulletin is doing that specifically angers FF - or something FF is configured to do that becomes overly sensitive by vBulletin.
Personally, since FF started this fast-tracking of updates and version releases, I've found it to be less stable. I have to use it at work for a few things, and find that some things have just been completely broken by various updates over the past 2 years or so.