Changing the mail program used in Firefox
From MozillaZine Knowledge Base
Firefox normally uses the system default mail program for sending mail, such as when you click on a "mailto" link (example) or use the "Email Link" menu option ("Send Link" in Firefox 15 and below) to send an e-mail with a link to the current web page. This article explains how to choose a different mail program and how to fix problems sending mail in Firefox.
Choosing a different mail program to use in Firefox
If you want to use a different mail program in Firefox, open the Applications options/preferences window (" Tools -> Options -> Applications"), select the mailto entry and use the drop-down menu to change the action, as shown below:
"Use other" allows you to select an installed mail program that isn't listed in the drop-down menu:
On Windows, for example, if you want to use Mozilla Thunderbird and it isn't listed, click "Use other...", browse to the folder where Thunderbird is installed (usually C:\Program Files\Mozilla Thunderbird) and select "thunderbird.exe". "Use Mozilla Thunderbird" will then be listed as the "mailto" action and will remain listed as an option in the drop-down menu, even if you later change the mailto action to one of the other listed options.
In addition to installed mail programs, Webmail services (Yahoo! Mail and Gmail) can also be selected. If you want to use a Webmail service that is not listed, Firefox extensions to launch Webmail are also available.
Clicking "Application Details..." opens the "Application details" window, where mailto actions that can be used in place of the default mail program are listed. To remove an entry from the list, select it and click the "Remove" button.
Mail actions do not work
If you click on a "mailto" link, or if you select "Email Link" from the Firefox menu ("Send Link" in Firefox 15 and below) and nothing happens, try these solutions:
Reset mailto preference:
Delete or rename the mimeTypes.rdf file:
Reselect the the program to use in Firefox for mailto actions:
The Google Toolbar for Firefox includes a "Send with Gmail" feature that changes the mail program used in Firefox to Gmail, regardless of what is set in Firefox options or your system default mail program. It is recommended that you do not select the "Send with Gmail" option in the Google Toolbar. Firefox already has a "Use Gmail" option available by default in the Applications listing for mailto actions (see above). More importantly, uninstalling the Google Toolbar extension with the "Send with Gmail" feature enabled may leave you with Firefox opening nothing at all for mailto links. You can resolve this issue by resetting the Firefox user preference network.protocol-handler.external.mailto to the default "true" value in about:config (see above).