Firefox - Mozila Turn off absolutely all caching

Disable Browser Caching

To turn off the memory and disk browser caches completely, set these preferences:

browser.cache.memory.enable /*
set to false, default = true */
browser.cache.disk.enable   
/* set to false, default = true */

Turning these preferences off guarantees that Firefox will make an HTTP request for every URL it is asked to display.

Firefox does not use HTTP HEAD requests, at least not for the retrieval of web pages; it uses GET requests. Every URL can therefore be made to return a full copy of the resource (the web page) in question. Embedded images and other URLs are, of course, retrieved in separate GET requests. It is possible that web pages or Firefox extensions might issue their own HEAD requests, though.

GET requests never add any HTTP If-Modified-Since headers if local caching is turned off. Such headers are only sent when local caching is enabled (the default) and if a currently cached copy of the requested page was originally delivered with a Last-Modified header.

The golden rule of standard Firefox URL loading is this:

Everything is always refreshed from the Web if local caching is disabled or if the Reload (bypass cache) command is run.

Reload (bypass cache) is run with Shift-F5 (Shift and click Reload on the Macintosh). Just left-clicking the Reload button is not enough, because Reload obeys HTTP caching protocols (see the next section). Clicking Reload is enough if local caching is turned off.

Even less reliable is pressing Enter or Return in the Location bar. In that case, if local caching is still on, Firefox might not issue an HTTP request of any kind. Change this preference to 1 (one) to fix that:

browser.cache.check_doc_frequency 
/* 1 = everytime. default = 3 = as needed */

There is, however, an exception to the golden rule. The exception applies to uses of datasources, an advanced XUL programming topic:

The content of datasources persists until they are destroyed or until Firefox shuts down.

Datasources are an internal feature commonly exploited in XUL applications. Datasources are also used deep inside bookmarks and elsewhere.

Switching Firefox into Offline mode also affects page caching. If Firefox is in offline mode, it will attempt to use cached copies of web pages, even if the network connection is still available and it is only Firefox that is offline, not the network.