FF implements what they call 'smart sizing' on Android. The basic principle is the larger the amount of disk space available (on Android they use disk caching) the larger the cache size. You can inspect this dynamically on your device in 'about:cache'.
I am violently critical of FF's current direction but to be fair to FF the reloading of tabs is a compromise to deal with how little disk space there usually is on Android (because the OS itself hogs large chunks of it in most Android devices).
* Scroll lag/tearing on many websites which doesn't occur with Chromium engine
* Reloading of tabs when I return to them, vs. leaving them as-is until I request a reload. This is abominable behavior.
So I installed Vivaldi and then blocked the domain on my pi-hole to prevent the daily checkins the browser attempts.