I’ve been using Firefox for 3 years now, and had a rare problem the other day that I’d thought I’d let other people know about.
The title pretty much sums up the problem – no matter how many sites I went to, the back button always appeared greyed out. I use keyboard shortcuts a lot so wasn’t that much of a problem – I would just hit the ‘backspace’ key to return to the previous page. Obviously it was still annoying because you could only go back one page at a time so it required a bit of urgent attention. There was also a strange problem where hitting the enter key to submit a form (rather than click the submit button) would perform the task of the ‘tag’ button on the del.icio.us toolbar plug in.
It was actually very, very easy to solve and simply involved clearing my internet history, cache, cookies etc., and restarting Firefox.
If you have a problem similar to this, then the excellent Firefox Troubleshooting Guide should help, but it just goes to show how rare problems with Firefox are.