If you're doing web development – building sites, writing html and/or javscript, writing WordPress plugins, then you need to be able to test your site and code out with different versions of a web browser.  Firefox is easily given its open source nature and versions of portable apps out there.  You can have both Firefox 2.0 and 1.5 installed on your system and launch the appropriate one. For IE, this is a much harder task as Internet Explorer installs dlls all over the place, performs upgrading checking, uses the registry, etc.

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