"A collection of web page copies stored on your computer's hard disk or in its random-access memory (RAM). Firefox accumulates these copies as you browse the Web. When you click a link or type a URL to fetch a particular web page for which the cache already contains a copy, Firefox compares the cached copy to the original. If there have been no changes, Firefox uses the cached copy rather than re-fetching the original, saving processing and download time." It is save to remove. Try clearing your cache: 1. On the menu bar, click '''Tools|Options''' 2. Click the '''Advanced Panel''' 3. On the '''Network Tab''' , click '''Clear now'''