Recently installed Latest version of Ubuntu...

Hi, I just got Ubuntu for my laptop. An envy notebook with i 7 processor and 16 gb of ram. Very good pc, however, when i open more than5 or 6 tabs in mozilla I sometimes get a lag, and its hard to close the tabs without it freezing up. Also, at one point I couldn't type smoothly, it would be really laggy. I had to completely quit out of mozilla. Then reopen it for it to go back to normal. Why is it so bugg/laggy??? I haven't done anything challenging on my pc yet...