While older versions of Adobe's Flashplayer for Linux made content like Youtube videos accessible via the /tmp filesystem, the latest versions hide these files from the user by exploiting a feature of unlink:
If the name was the last link to a file but any processes still have the file open the file will remain in existence until the last file descriptor referring to it is closed.
In other words, the flashplayer creates a new file in /tmp, deletes the file right away with unlink but keeps the filehandle open, so the flashplayer process may still access the file. This however, may lead to confusion - df reveals that the free space on /tmp is shrinking, while du doesn't show any growing files at all. One way to fix this issue is simple - use library preloading to overwrite the original unlink function used by firefox: Download the tgz-archive, unpack it and make it. If the previous steps were successful, you should now have a file unlink.so available. The last step is to tell firefox (or more precisely the dynamic linker) to use the unlink function from this file rather than the one from your C Standard library:
The LD_PRELOAD environment variable tells the dynamic linker to search for libraries in non-standard locations - in this case in our library file unlink.so. You might want to add an alias like the following to your environment, but for obvious reasons you shouldn't globally export LD_PRELOAD.
alias ff="LD_PRELOAD=/path/to/unlink.so firefox"
Yet there is one drawback with this solution: even if you close firefox, the files in /tmp will persist, so you may want to delete them manually from time to time...