How much cache should I give Firefox?

On a memory stick there is a finite number of writes allowed (a lot, but there is still a limit!) for this reason the cache is set as 0 on the portable version.

If you are running of a standard hard drive, then whatever you like - 50MB is plenty enough I would say.

Basically what rich said. On a flash drive, you want to limit writing as much as possible, which is why cache and session restore (among other things) are disabled in John's releases.
With a hard drive, you can enable these because the storage medium is much more robust. I have my cache set to 100MB.