Lots of .tmf files in the %TMP% directory
Using an older W2K Pro setup, Firefox 12.0. These files accumulate forever, apparently when a new page is loaded with custom fonts. Cure seems to be going to Tools -> Options -> Content -> Advanced Uncheck "Allow pages to choose their own fonts, instead of my selections above" That keeps the files from appearing, but obviously doesn't allow the custom fonts.
Run a disk check utility on your system and tell it to remove temp files.