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Will extensions that work in FF5 work in FF3? Are extensions backward compatible?

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I'm using FF5 but I have the need to test the compatibility of certain websites again FF3 (3.6.18). Do I need to uninstall FF5 and reinstall FF3 and all of the associated plug-ins or can I just install FF3 and run it?

I'm using FF5 but I have the need to test the compatibility of certain websites again FF3 (3.6.18). Do I need to uninstall FF5 and reinstall FF3 and all of the associated plug-ins or can I just install FF3 and run it?

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One way to test both Firefox 3.6 and Firefox 5 is to create a separate profile for each, so that they will both have their own add-ons and will not interfere with each other. See Profile Manager - Create, remove or switch Firefox profiles for some instructions.