After trying 4 or 5 things from this several-years-old page on Firefox’s support forums, I finally hit upon one that worked:
“For the heck of it tonight I clicked on Gmail in my calendar and it finally went to my gmail inbox.”
In my case, I just went to Google Groups, and clicked the Gmail link from the top navbar. Lo and behold – it works!
(this is after a week of having no way to access Gmail from my main web browser)
Looking at the problems other people have had, my guess is that Google’s code for running Gmail has some illegal (i.e. breaks-the-standard) bits in there that try to get around the browser standards by doing silly things with caching. Some of those are … fragile, perhaps … and tend to break easily. Over time, Google has added more and more unnecessary “features” to that code (e.g. I often have to wait for “connecting to google chat”, even though I have google chat permanently disabled and will never EVER use it) … there’s a *lot* of code in there these days; lots that could go wrong!
Normally, a Refresh of a page would fix this – that’s how the WWW was designed in the first place, as a core feature – but Google’s (my guess) playing so fast-and-loose that they’re *also* (deliberately? or accidentally?) bypassing the refresh. I can imagine several well-meaning reasons they might do that, but in the end I’d rather they stuck to the standards, instead of creating these “breaks permanently” problems for people.
And, of course, there’s no Google support for this problem. Once it strikes you, GOOD LUCK! (you’ll need it)