If you block Flash and images and have an option to turn them on as and when required, then you'll multiply the amount of online browsing you can do for the same MB/GB limit. Because majority of those bytes are always eaten up by the images and flash - basic internet is very low on bytes.
Firefox extensions I used when I had a limited account : Flashblock, and ImgLikeOpera. Now the latter is getting older, here's another one that works and blocks both:
Image and Flash blocker - https://addons.mozilla.org/en-US/firefox/addon/image-and-flash-blocker/
Flashblock puts a handy play button on top of web video etc so you play it only when you want to.
It's also a good idea to block ads off completely to minimize wastage of your bandwidth, plus avoid evil cookies and this makes your pages load faster:
Opera browser has an inbuilt feature to be able to block/unblock images at a click, and is quite fast - on par with Chrome and isn't as heavy as Firefox. The button sits in the lower corner somewhere.
I don't know what's there for google chrome and IE : don't ask!