Make your firefox back button supper fast.
Go to url: about:config
browser.cache.check_doc_frequency documented on
Set how often to check accuracy of cached documents
0 : Once per session
1 : Each time
2 : Never
3 : When appropriate/automatically
1 is the setting you’re looking for. There will be a small performance hit, because the browser wiil always connect to the server, but it won’t be substantial, because it won’t re-download a file if it hasn’t changed.