Firefox:
- Easy and Fast
- Customizable
- Lots of Unique Features (Private Browsing, Tabs Grouping etc)
- Lots of Add-Ons and Plugins
- Best for Development (FireBug, Web Developer Toolbar, SEO Plugins, Color Pickers etc)
- Provides best support for Editing and Testing Live Sites.
- Very good support Community
- Reliable and stable
- Very Good for Testing New Web Projects
and many more.. chrome is also good, but it will always remain behind firefox. For example, Firebug was primarily a Firefox Plugin, now they gave support for Chrome, but we used FireBug in FF for more than a year before it was out for chrome..