FireFox is deffinatly the most popular Open-Source program available. It is a great Internet Browser. If you look at the new version of IE it is apparent they modeled it after FireFox.
These are the add-ons I recemend:
FasterFox: Makes Pages load considerably faster.
Ad-Block Plus: Say good-bye to Ads.
StumbleUpon: Find pages that fit your interests.
Down Them All: A mass downloader.
ForecastFox: Weather integrated into your browser.
Aero Fox: A dark theme.
Download FireFox At: http://www.mozilla.com/en-US/firefox/
Download Add-Ons at: https://addons.mozilla.org/