[Integer] - This setting determines how often Firefox checks the page you're viewing against the cached version it holds. By default (a value of 3), Firefox automatically checks and only reloads the page if it seems outdated; a value of 2 will never check and always load the cached version (not recommended); a value of 1 will always check for a newer version of the page and reload; a value of 0 only checks once per session. In most cases the default of 3 is fine, but you may wish to set this to 1 if you want to ensure that Firefox always checks for and loads up the latest version of a page, though it may reduce browsing speed.
browser.cache.compression_level [Integer] - This setting determines whether Firefox compresses the data in the disk cache, reducing the size of it. The default value is 0, which means compression is disabled. A value of 1 to 9 enables compression, with the higher the value, the greater the data is compressed, but the potentially slower browsing may become.
browser.cache.disk_cache_ssl [Boolean] - When set to True, which is the default, this setting allows the caching of secure web pages in your browser disk cache. This speeds up loading of commonly-visited secure pages, but is a security risk and for that reason you may wish to set this to False.
browser.cache.disk.parent_directory [String] *Create - Determines the path to the parent directory for the Firefox cache, usually held under your profiles directory (i.e. under "%APPDATA%\Mozilla\"). By specifying a new full path here (e.g. "D:\Users\User1\Documents") you can move the cache to another drive or partition, which may improve performance or enhance privacy. Note that if this value is being set directly in the Prefs.js or User.js file, the path requires double backslashes for every backslash instead of single ones (e.g. D:\\Users\\User1\\Documents).
browser.cache.memory.enable [Boolean] - If this preference is set to True, Firefox will use system RAM to cache certain data, such as images, improving performance. If set to False, Firefox will not use any RAM to cache, resulting in a performance drop. I recommend leaving this setting at its default of True unless troubleshooting a potential memory-related problem in Firefox for example.
browser.cache.memory.capacity [Integer] *Create - If the browser.cache.memory.enable preference above is set to True, this setting allows you to specify the amount of memory to use for caching data. A value of 0 turns off caching into RAM, which is not recommended as it can slow down browsing. The default value of -1 tells Firefox to automatically determine the size of the cache based on your physical RAM. Systems with 512MB of RAM will result in a 14MB cache, 1GB RAM = 18MB cache, 2GB RAM = 24MB of cache, and 4GB RAM or more = 32MB of cache. You can check the actual amount of RAM allocated for this feature on your system by typing about:cache?device=memory into the Address Bar and pressing Enter, then checking the value in the 'Maximum storage size' field. The default of -1 should be fine for all people, but if you want to set a specific amount of RAM to use for the memory cache, enter a value in KB (e.g. a value of 18432 = 18MB).
browser.cache.offline.capacity [Integer] - Web applications can be stored locally for offline use, as covered by the 'Offline Web Content and User Data' setting in the Firefox Options. This setting determines the maximum amount of space they can take up in the offline cache. The default value of 512000 in kilobytes corresponds to 500MB. In general there's no need to alter this as it will only be used if you install an offline web application, and is not to be confused with the Firefox disk cache for normal web content. If you wish to turn off the offline cache altogether, set the browser.cache.offline.enable setting to False.
browser.cache.use_new_backend [Integer] - If set to 1, this setting enables a newer form of the caching system in Firefox, designed to provide improved performance and be more crash proof. I recommend enabling this option (1), and only disabling it (0) if you experience problems. Note that using the new caching system will mean your cached files will be stored under a different directory from the regular cache, namely \Users\[username]\AppData\Local\Mozilla\Firefox\Profiles\[profilename]\cache2. The maximum size of this cache is determined by the browser.cache.disk.capacity preference. Note that as of Firefox 32.0, the new caching system has been enabled by default. However it appears that a browser.cache.use_new_backend_temp preference has been added and enabled instead to implement the new caching system.