Firefox is stable and secure, and is ultimately customizable with extensions.
Extensions are add-ins written by regular people that add functionality, pizzazz, and pure de ole fun to Firefox.
For security, download NoScript, Flashblock, Adblock Plus, Adblock Filterset.G Updater.
If you use Yahoo mail or Gmail, download Yahoo! Mail Notifier or Gmail Manager.
There's hundreds, if not more than a thousand, extensions, and all are free. Just click (in Firefox) Tools > Extensions > Get More Extensions, and start browsing.