Since the beginning of the internet in the early nineties, browser market share has wildly fluctuated. As an excellent article on the browser wars at evolt.org explains:
In the First Era of browser history Mosaic and the other early browsers ruled. The Second Era was that of Netscape dominance. Microsoft's challenge to Netscape marked the beginning of the Third Era, the Heroic Age of the Browser Wars. Netscape's bleeding to death marked the start of the Fourth Era of Explorer dominance.
We are now officially in the Fifth Era, where Explorer starts to lose its dominance to an Open Source (freely available) competitor, Firefox by the Mozilla Foundation. 
Here are some reasons to switch:
- Inbuilt Popup Blocking
- Tabbed Browsing (viewing more than one web page in a single window)
- Privacy and Security (no AcitveX controls, thus much safer than IE)
- Intelligent Search (Google Search is built into the toolbar, and there's a great “find in page” functionality)
- Lots of neat extensions, like local weather and controlling your music center at the bottom of your browser window 
At its peak, IE had 93-94% of the market. As of April 2005, there have been more than 40 million Firefox downloads and Explorer's share is eroding fast. Microsoft originally announced that they would not update IE until the next version of Windows, but now they are scrambling to release an IE 7 beta version by this summer.
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personally…..i feel there’s not much diff in any of the browsers…..any one will bust their humps for their subscribers….