AFAIK, the default firefox flatpak configuration has the cache path in ~/.var/app/org.mozilla.firefox if it is a user install, which is mounted/passed into the running container instance.
You can change the cache path in about:config via browser.cache.disk.parent_directory and if you want to use a path outside of the flatpak container, you can use flatseal to create a filesystem overwrite.
Hope these infos help.
It turned out to be a lot more complicated https://github.com/JimmXinu/FanFicFare/issues/985 and not at all a Flatpak fault.
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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