It seems there is no way to set the frequency of when FireFox3 checks for new files on the server. If you are a web developer and are testing your code, you really want the browser to check for changes “every time” you refresh the page. You can always force the clean refresh by holding down the Command key (aka Apple key) on Macs or Shift on Windows.
There is a less painful way to do this in the advanced settings of FF3. Just open up about:config and look for browser.cache.check_doc_frequency. The value can be:
0 once per session
3 when out of date (default)
I recommend changing this value to 1 if you are developing and testing code.
You will then see lots of 304 not modified messages, and get 200s for your changed files.