A google search will produce a pretty hefty list of reasons why you should switch.

Personally I will be using FireFox indefinitely, but a short list of reasons why to switch is FF is more secure, has more features, updated continuously, more configurable, and more up to date with current web standards.
Oh and don't forget that is open source, so its not controlled by a company that cares more about making money then producing a quality product.

The only reason that the majority of the internet uses IE is because it comes default with a new computer, and many many people are not aware that they can use another program besides IE to use the internet.

The only big con I can think of is some websites are built specifically for IE, and they won't run in FF. Its not many, the only ones I come across are University run websites, and were designed specifically for IE, (they aren't simple websites, one for instance is used to install software onto your computer for your classes) so its not websites you will come across when using a search engine.

And its not like when you make the switch you can't go back, you can still use both browser, even at the same time.

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