How can I take screenshots of full threads?

I downloaded an addon for Firefox but whenever I take a screenshot of just the main page of any board it is like, 3.5 MB. What program can I use to take images of full threads where they won't end up being  3.5 MB?

Also, I'm new to 4chan BBS, I'm usually on the imageboards, so if this is the wrong board I'm sorry.

Thanks.

screengrab addon for failfox should set you right OP. It produces optimized .pngs of entire websites. ~38kbs in size.

>>5
optimized .pngs of entire webpages