https://forums.mozillazine.org/viewtopic.php?t=2394533

https://kb.mozillazine.org/Browser.cache.check_doc_frequency

Background

When a page is loaded, it is cached so it doesn't need to be downloaded to be redisplayed. If the page changes after a previous visit, you may want to redownload it anyway to get the updated page. This preference controls how often to check for a newer version of a cached page.
Possible values and their effects
0

Check for a new version of a page once per session (a session starts when the first application window opens and ends when the last application window closes).
1

Check for a new version every time a page is loaded.
2

Never check for a new version - always load the page from cache.
3

Check for a new version when the page is out of date. (Default) 

Check it in about:config.