Recently, firefox screamed at me because my /tmp “partition” was full. This stuck me as odd because I did not create a manual /tmp directory. After running “df -h”, I found that ubuntu had created a /tmp virtual partition and set it’s size at 1MB. This also stopped videos on Youtube from playing as 1MB fills up very quickly.
I found the fix at http://ubuntuforums.org/showthread.php?t=677185&page=3#22
Open up /etc/mtab in your favorite text editor with root privledges (ie “sudo vim /etc/mtab”). And remove the line that dictates your /tmp directory. Ubuntu will now use a directory under /, like a normal OS would do.