What is the latest version of firefox suported on this tablet?

I’ve had similar problems with older tablets where the apps slowly become the real limitation. If the tablet can’t get any newer Android updates, I’d probably try to find the newest Firefox version that still works on it and see how it behaves. At some point though, the old OS is going to be the limiting factor rather than the browser.

Will Firefox work on my mobile device? says you need Android 8 since a few months ago, and your maximum version would be Android 7 presumably. So you're stuck on some older Firefox version before v144.

There are alternative Android installations that might be compatible with your device, and keep you more secure and up–to–date, but I'd suggest searching for expert forums on that specific topic first how viable that is.