The speed of your internet connection and your ISP's limit will not change no matter which browser you use.
Imagine that your ISP is like a mail carrier. The limit is how much mail they agree to send to you. So if you download too much, then they'll stop sending, send slowly, or charge you money.
The 50 MB that your browser uses is different. To do work, a program needs a cpu to 'think', and some memory to keep track what it's thinking about. (So no, 50MB does not get used.)
A browser is a bit like a butler who gets your mail and needs to use a notebook (memory) to make calculations about where on the screen to draw this text, and where to draw that image. If a browser uses too much memory, everything else might slow down. Usually it's not a big deal. The more important things are the features, and how fast it draws the screen for you.
IE doesn't use up a lot of paper (its memory use is smaller), but it's a bit old and crotchety. It can't do as much as the newer browsers, and it sometimes forgets to lock your doors at night, but many users like IE because it's been with the family for so long - it's familiar, and that's good.
Opera is like a butler trained in some European country. It's very clean, very usable, very secure. It's fast, and it doesn't use too much memory. It has some neat features.
Firefox is... I have no analogy. With the brand new version, it does pretty much anything Opera does, and with add-ons a whole lot more (it'll remove ads, block scripts, help you search). The last version had a problem with memory, but that's been fixed now:
There are updates for Opera too though, and Firefox and Opera will be very close in terms of performance for a while. IE is and will probably remain behind.
The main reason people say Firefox is better is because of the community. There are more people to help you use Firefox, and more people to fix it. Opera does not have more features than Firefox, because Firefox can get add-ons to do almost anything you want.