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Associate Protocols with Web Application was introduced last year for Firefox 3 that is a common way to link to mail addresses using the mail to Protocol.
Yahoo Mail was the only the webmail service to be accessed when Firefox 3 was introduced.
Now you can set your Gmail as your default email client in Firefox. Follow the following steps:

•Go to Options in Tools.
•Choose the Application tab.
•Search for the Mail.
•Then select Use Gmail from the list of actions associated with the mail to Protocol.
•Finally click OK to save the settings.
Moreover, you can set Gmail as your default mail client for your computer and not for only Firefox.

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