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Open Firefox and pull down the menu in the upper left corner (if you are using a different/earlier version of Firefox, go to the Tools pull-down menu and choose Options).
Go to Tools and select Options, then Options (again):
In Options, choose the Applications Tab and locate "mailto" in the list under Content Type:
Pull down the menu where it currently says "Use Novell GroupWise ..." and Select "Use Gmail"
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Before you close the Options box, click OK to save the setting.
This will enable Firefox to choose your Gmail account when you click on any link that should point to email.