From MozillaZine Knowledge Base
When a page is loaded, it can be cached so it doesn't need to be downloaded to be redisplayed. For e-mail and news, messages and attachments are cached as well. This preference controls whether to use the hard drive for caching purposes. browser.cache.disk.capacity controls the maximum amount of disk space to use.
Possible values and their effects
Store cache on the hard drive. (Default)
Don't store cache on the hard drive.
First checked in
Present since at least 2000-10-30. Exact checkin date unknown.
Has an effect in