Firefox 3 allows you to change the default mail client associated to the mailto: links. Originally, if you used a web mail client you were stuck. Firefox 3 has fixed this by allowing you to create an association to Yahoo or gMail.
Leverage Yahoo Mail
1. Select — Tools > Options and select the applications icon on the top.
2. Scroll down to mailto and select Yahoo Mail
3. Click ok
Note: You will be prompted to type in your user name and password if your not already logged in.
Leverage Google Mail
Google doesn’t show up in the list, but its simple to configure.
1. In the address bar type about:config and click through the warning message.
2. Scroll down to or in the filter box type gecko.handlerservice.
3. Locate gecko.handlerservice.allowRegisterFromDifferentHost. double click to set to TRUE.
4. You will be prompted to accept gMail as your mailto client. Select Add Application Button.
Note: The next time you select a mailto link you will be prompted to select the client to use. You can select “remember my choice for mailto links” to set gmail as your default.