Clearing the cache
Just like Internet Explorer, and others, Firefox keeps cached files of previously visited web sites so that when they are visitied again, they will load much more quickly. However, over time, you may have quite a buildup of files in the browser cache storage. It is a good idea to delete these files from time to tim to keep your system running smoothly. Here's how:
Click Tools>> Options
In the Options windows, select the Privacy icon
Click the Cache tab
Select the Clear button
Note: You have the option of changing the amount of disk space that Firefox uses for the cache storage. The default value is 50MB or 50000KB.