IIRC MIUI has for quite a while been tuned very aggressively in terms of memory management. That is to say, it’s probably not the browser but the underlying OS that is pruning processes to keep memory free when it really doesn’t need to.

I had some issue with miui. There is nothing we can do.

As far as I know it’s an issue with Firefox, always been like this.

Unfortunately your best bet is using a different browser on android.

Do you have the same issue after rebooting your device?

Android’s memory management may be struggling with a lot of cached resources in RAM that are being used frequently, besides Fennec. (Cached resources’ memory space may show up as “available”, but in reality this space is in use - run cat /proc/meminfo in a command line on your Android to see how much RAM is “free”)

When this happens, Android pretty much sends memory warnings to apps as soon as they are moved to the background, forcing them to save the bare minimum data needed to resume, before they are killed by the runtime. Fennec in this case is just saving your active tabs, but not the form data within. This behavior cannot be changed

Only fix in this scenario is to reboot your device AFAIK

My workaround on this is to split screen firefox and the app I need to open (ex. password manager).

Firefox for Android straight up sucks. Sorry but it’s true. It’s heavy and slow with a bad ui. It basically has one and only one benefit. Add-ons. That’s it. It’s even worse on tablets. It’s basically the same app but bigger. I bet Android users make up most of Firefox mobile users but they can’t make a proper app.