Here is some explanation from firefox dev from two years ago https://www.reddit.com/r/firefox/comments/md63k4/comment/gs8mo2t/
The answer is complicated.
Mobile operating systems don't have swap space. When running multiple apps, a mobile OS needs to be more aggressive at freeing memory than desktop operating systems do. They do this by terminating background processes. The OS uses various heuristics to decide which process(es) to kill.
The problem that you're seeing is not intentional on our part; there isn't something in Firefox that says, "unload everything whenever I go into the background." Instead, it is caused by the content processes being terminated by Android itself.
We know that Chromium-based browsers seem to be working better in this regard. We do not yet have a clear picture about what specifically is causing our content process to be a frequent target.
We're in the process of collecting additional telemetry to help us diagnose this. I've also landed a patch that helps to clean up content process memory usage when Android tells us that it needs memory, in the hope that it will reduce the likelihood of a content process termination. We're also testing Nightly with multiple content processes enabled, which may help.
At any rate, I wouldn't call this problem an intentional design decision, nor would I call it solved. We're doing what we can to learn more about it and get it fixed at some point.
Sounds to me android is by design letting chrome based browsers and killing everything else.