Firefox is light years ahead of IE7.
If you click tools->add ons, and then click the little link in the corner saying 'get more extension' (or something to that effect), then you'll be taken to a site where you can add extra add ons, as Ethel says.
Ones to search for and install:
Adblock Plus
Fasterfox
No Squint
Plenty others that I use too, but those 3 are almost essential.
Adblock blocks adverts in web pages, stopping that flashy annoyance. Fasterfox speeds up downloads and general web browsing (optimising for your connection basically), and No Squint is great if you ever visit websites where the text is a bit too small to comfortably read. You can increase the text size with ctrl+[plus key], but this is for all sites you visit. No Squint makes a per site setting, so the one site that you always visit and always increase the font size will automatically be set to the nicer size when you visit it.