The last time I had this happen to me was two weeks ago and I still haven't completely figured it out just yet. With that said, here's what I did and maybe it will work out for you. There might be a better, more complete and less destructive way to do this but this doesn't happen very frequently for me to test.
It's a two step process.
First, get Firefox to forget about the site, everything. You can do this by going to your history, right click on a link to a page on the domain you want to forget, and select
Forget About This Site. After you do this, close Firefox.
Now you want to kill any traces of the old page from Windows' DNS cache. Go to the command prompt (Win + R,
cmd) and enter the command
ipconfig /flushdns. After this close it as we're done.
The next time you start up Firefox and visit that site, it should now (hopefully) be the most recent version and not a cached copy.