Given that your preferred and normal browser works fine here, it would seem possible that the problem lies with the other forum - particularly as you can access it via the IE tab in FF.
That's exactly what I was thinking. I have FF set to clear cookies at the end of each session, and the cache is cleared regularly when I run CCleaner. I'm typing this on my parents 'puter and when I logged on to the problem site just now I find that the "Edit" button is missing from all my posts! I'm actually a Mod on the site and posted in their section, but all I got was suggestions that I should change my firewall, A/V etc. There are regular complaints about forum problems but I'm beginning to think they don't want to admit things may be less than perfect...
I'm not going to pull my (few remaining) hairs out by embarking on a session of system restore and uninstalling/reinstalling various software - computers are supposed to help us not be a source of endless frustration.
Thanks anyway.