All the suggested benefits listed above could easily be implemented in a script. For instance, one could design a script that checks the amount of disk space at logout. If the amount of disk space remaining is less than X (where X is value predefined by an administrator), the script could _suggest_ that corrective action be taken and allow the *user* to _decide_ what he/she wants to delete/move.A better option in this case is to reduce the default size of Mozilla's cache or expand the size of the quota for each user to deal with the added space requirements.
If you are concerned about disk usage from caches, you can always create a script that removes the cache(s) when the user logs out.
That's not the right thing.. that way you loose caching effects around
logins even when there's plenty of space.
There's quite a lot of data -- at least on my systems -- that can be
removed with "only" loss of performance...
1) browser caches
2) package lists, downloaded packages
3) object files
heck, if you know you have reliable network connection 4), you could
even mark stuff like /usr/bin/mozilla elastic, and re-install it from
the network when it is needed... Doing anything more complex than 1)
requires extensive changes all around the kernel and userland, and
you'd probably not call that system unix any more.