Where's the cache?

To direct the cache to disk just modify the procedures mentioned here to suit what you want to achieve: https://thetechjournal.com/how-to/tutorial-how-to-move-your-mozilla-fire... -- study it for a few minutes and you'll get the idea.

However, if you don't want the cache to be written to Firefox Portable's default location for the disk cache then in about:config search for the preference browser.cache.disk.parent_directory. If the preference does not exist then create it as a new string. To do so right-click in an blank area of the about:config preferences display and select new > string For the new string give it the preference name browser.cache.disk.parent_directory Once that is done enter the path for the cache. So, for example, let's say you wanted the cache on partition "W:\" then you might give the following path: W:\\FIREFOX\\ (the path needs double back-slashes, not single). Firefox will now create the disk cache in the location you specified (in this example that would be W:\\FIREFOX\\cache2 (but you shouldn't specify the cache2 part of that path).

Just experiment with this a bit and you will get it working fine. That said, I don't see the value of putting the cache to disk on a portable Firefox setup. Also in a fully installed setup this would be questionable from the point of view of hits to disk as the cache gets built up over the course of a day of browsing. For my fully installed Firefox I do have the cache set to write to disk. However, the disk I have it write to is a ramdisk -- so no cache content ever gets written to my SSD. Why do I do this, after all writing the cache data into memory would still prevent writes to SSD? Well I just love at the end of a day looking in the cache folder just before I shutdown Firefox and watching the several thousand (yes several thousand) potential writes to the SSD just vanish when I shutdown Firefox. It's my idea of fun but it does serve a serious purpose -- reduce writes to SSD. If I didn't enjoy that so much then I would just have Firefox, installed or portable, write its cache data into memory.

EDIT: For default install of Firefox Portable, and with disk cache enabled, the default cache location would be: [WhateverPath]\FirefoxPortable\Data\profile\cache2, Do keep in mind that if you specify your own path to the cache then you can seriously break the portable nature of Firefox Portable. So best advice, if you really do want to write the cache data to disk, is not to specify an alternative path for the cache -- just leave things as they are.