These are my favorites. Firefox is so superior to internet explorer. It's sad many people don't download the extensions.
Google Toolbar For Firefox: Adds Google Toolbar to firefox. Includes an option for spellcheck for when you're blogging or sending emails. Google search is the best search engine, use it! We have a link to firefox with googlebar in our help us free page.
Customize Google: Allows you enable several useful google features to enhance browsing. Watch their presentation.
SiteAdvisor: Shows which sites are bad, as determined by the sites downloadable files and how much spam they send when signing up. Site includes a presentation.
PDF download: PDF's are annoying to load. It takes awhile for adobe reader to load and sometimes it even crashes. Some sites dont mark links as pdf at times. This program tells you when it's pdf file you're clicking and even offers to load it in html format.