What do you mean by crash? Most browser games run on a potatoe. Did you get any error message, or did the browser window just dissappear?
Did you try with less addons or a different browser, as is the usual way of starting to find issues.
Also about:performance, type in as a website on ff.
If you say tab, does this mean you have different tabs open with different games? It has been a while, but when I did that, sometimes games would crash, when specific games where loaded. Some games did not play nice, so to speak. So I could have game a b c open, but if would combine game d with either of them, both would crash.