If you mean a current version (i.e. not 68 or earlier), then it's not really possible – both because I think there's no option to configure Firefox's tab unload

If you mean a current version (i.e. not 68 or earlier), then it's not really possible – both because I think there's no option to configure Firefox's tab unloading itself except by recompiling, but also because now that Firefox loads tabs in separate content process, those separate content processes are an easy prey for the OS low-memory killer. Even the most-agressive memory optimiser won't kill the current foreground app itself, but anything else including content processes for unused (from the OS point of view) tabs may be fair game, even if that doesn't actually improve the ultimate user experience... If your phone is known for aggressive memory management (https://dontkillmyapp.com/ https://dontkillmyapp.com/), maybe exempting Firefox from its memory optimisations (if that's possible) might help a little, but no guarantees.