The user agent switcher for firefox can be a really useful tool, generally firefox support is good, but some web based applications reject it because they don’t recognize the agent string, with this plug-in you can fool the web server into thing you’re using IE.

This tool bar is really cool, it allows you to outline and label important objects, verify code and much much more, highly recommend for anyone who does web development !
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If you use google then this extension is gonna be great. I came across it because it allow you to add https to your gmail so not just the username and password is encrypted, but it does sooo much more than that !
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