Is there an extension for Firefox which I could use to schedule so that FF would clear the cache every x minutes or hours?
Afaik, cached files are stored in a Cach-dir, in a hidden dir in your home.
would find files there, older than 120 minutes. With a switch -delete you would get rid of all the older files:
but don't hurry, I didn't investigate some files, which seem to be meta-files:
maybe they need to be in sync, so you should do an backup, before testing what happens on deletion. The files seem to be partly binary, and not trivial to understand.
If all goes well, you can write the command to your crontab with
for repeated cache-cleanings.