The Google toolbar already provided a feature for this, but you can
also do it right in the latest version of Firefox*: associate “mailto”
links in web pages with Gmail, so that when you click on e.g. bill@gmail.com
you’ll be instantly forwarded to create a new message in Google’s email
client. Open Tools -> Options -> Applications, and look for the
entry titled “mailto”. In the dropdown box, pick Gmail. URL: Associate Email Links With Gmail in Firefox (Natively)
...
Anytime
The Google toolbar already provided a feature for this, but you can
also do it right in the latest version of Firefox*: associate “mailto”
links in web pages with Gmail, so that when you click on e.g. bill@gmail.com
you’ll be instantly forwarded to create a new message in Google’s email
client. Open Tools -> Options -> Applications, and look for the
entry titled “mailto”. In the dropdown box, pick Gmail. URL: Associate Email Links With Gmail in Firefox (Natively)
...