To clear Firefox's cache of web page contents:

Tools > Options > Advanced > Network > Offline Storage: "Clear Now"

Flash may store information outside of Firefox's cache. You can see whether any sites are consuming more than their share here: http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

You can check the Virtual Memory settings here: Control Panel > System > Advanced > Performance > Settings (Advanced tab)
Windows can manage more memory then is physical available and that memory is stored in a page file on your hard drive.
If the size of that file is not large enough and all available memory is in use then you get the Virtual Memory is low warning.
Make sure that you have at least 1 GB or the recommended setting for the Virtual Memory.
If you have less than 1 GB physical memory installed on your computer then if possible add more memory.

• http://support.microsoft.com/kb/555223 - RAM, Virtual Memory, Pagefile and all that stuff
• http://en.wikipedia.org/wiki/Virtual_memory

My Firefox 7 "Tools" has no selection named "option."

If you are not on Windows then instead of Tools > Options use:

• Linux: Edit > Preferences
• Mac: Firefox > Preferences
• Windows: Firefox > Options or Tools > Options

Press F10 if the menu bar is hidden on Windows and Linux