Sounds like you might have run out of disk space on the firewall. Try opening a console session and runing 'df' - or 'df -h' for human-readable output - and see what space is left on your root partition. The /var/tmp folder is normally located on that partition.
If you have run out of space, check to see what packages you've installed, if any. Ntop, for instance, can use up a lot of disk space quite quickly if you have a lot of traffic running through your firewall.