No longer a fledgling upstart, Firefox is the gold standard of alternatives to the still-dominant Internet Explorer. This browser is full-featured, secure, and lightning fast--although competition is strong and it can no longer be said that Firefox is the fastest browser available. Its killer selection of add-ons remains strong, however, with built-in support for the next generation of themes, called Personas.
The latest update to Firefox 3.6.6 fixes security holes and solves some problems with plug-ins crashing (for example, most notably, a Flash or Silverlight crash now won't take Firefox down with it). Upgrading is highly recommended.