Hello,

Is this problem happening with Firefox only? Did you try any other browser for comparison? Plese try getting a screenshot for it as it will let us know what kind of problem is it. Are you facing this problem from the start?

Try cleaning the phone memory(internal) if nearly full, clear the cache memory and free RAM a bit. If it does not work then try clearing cookies or reinstalling the browser.

Have tried clearing memory, cleaning cache, even reinstalling FF. Does not occur with other browsers Chrome, Adblock browser.

The best thing you could do is get a update to at least Android 5.1.1. or restore factory reset and update your tablet. Delete everything you did not use. It may free up space and run faster than before.