Prevent "document expired" message and just show the expired page?

The Firefox engine does not (or should not) cache the last state of documents, only the HTML document itself if the server allows caching. Opera had (or maybe still has) this feature, but it has caused many security headaches for website operators.

RFC 2616 states:

13.4 Response Cacheability

Unless specifically constrained by a cache-control (section 14.9) directive, a caching system MAY always store a successful response (see section 13.8) as a cache entry, MAY return it without validation if it is fresh, and MAY return it after successful validation.

A lot of sites nowadays employ the cache-control header on all responses generated by the server to avoid caching problems. Even if you get Firefox to give you the document in the former state, on the majority of sites if wouldn't or shouldn't work anyway. The safest and most future-proof solution would be to ask the website owner to code their website properly.

To get rid of all these messages add the following PHP lines to the beginning of all your scripts:

header("Expires: Sat, 01 Jan 2000 00:00:00 GMT"); header("Last-Modified: ".gmdate("D, d M Y H:i:s")." GMT"); header("Cache-Control: post-check=0, pre-check=0",false); session_cache_limiter("must-revalidate");

The answer is on https://support.mozilla.org/fr/questions/1010942 : just check Tools->Options->Override Automatic Cache Management.