This also happens on my Samsung tablet using LineageOS, so it's not an issue specific to Xiaomi.

I have quite a few Extensions installed, I'm not sure sure if that could be relevant.

I’m experiencing something very similar on my device as well. Even with battery restrictions disabled and the app set to “No restrictions,” Firefox still reloads tabs almost every time I switch to another app for a short while. It’s especially frustrating when you’re filling out a form or reading something long and then everything refreshes and progress is lost.

It feels like memory management is a bit too aggressive compared to other browsers. I understand Android can be strict with background apps, but other browsers seem to keep pages in memory longer under the same system settings.

Would be great if there was an option to make tab retention less aggressive, even if it uses a bit more RAM.