Comment 1 for bug 234661

In Most cases where these data losses occur it will be due to the Disk Cache
saving enough data during a session to bump the disk up to the full point, or to
the user downloading enough files to use remaining space.

The shut down routine should check for disk sapce and if it is full the disk
cache should be emptied to free up some space. This is also a data loss, but
cache loss is far less severe than losing bookmarks, passwords, etc. Emptying
the cache should prevent loss of critical data in most cases, the exception
being people who have set their disk cache to zero size.