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