Hi Joey, do you install Firefox directly from Mozilla or through your distro’s package manager?
There seems to be a lack of communication between Firefox’s internal updater and some package manager updaters that leads to unpredictable updates. Or should I say, package managers used to effectively disable Firefox’s built-in updating and recently it has stopped working in some distros.
I do not follow the details on that. If you want to keep using your distro’s build of Firefox, please check with them on whether this is a known issue and has a solution.
For the build directly from Mozilla, use the Preferences page to change update behavior. The about:config setting is obsolete on Windows and may be obsolete on other platforms.