Basically, I would like Firefox to behave as if all responses from example.com carried the "Cache-Control: no-cache" header. I have searched the Mozilla addons site and there is a boatload of hits for "add http headers" but on closer inspection they all modify just the request headers. Perhaps there is another way of doing this, or maybe there isn't :-(
I don't know how to do it for an specific site, but this is the equivalent setting to IE's "Check for newer versions of Stored Pages: Every time I visit the webpage" in Firefox:
Type in the address bar about:config, then search for browser.cache.check_doc_frequency. Set its value to 1, which means the website will always load the newest content every time you access it.