After clearing the cache, you might want to restart Firefox too (BEFORE reopening the site that's giving you problems).
To clear other stuff .. cache, cookies, offline web site data, site prefereces, etc.
In Firefox, go to:-
Edit > Preferences > Privacy
History > Firefox will: Remember History
Then click the blue writing that reads "clear your recent history"
Time range to clear: Everything
Offline Web Site Data
And click the "Clear Now" button.
Give it a minute or two to clear .. then restart Firefox.