Finally, Mozilla registered Firefox to handle mailto: URLs in Firefox version 64 in Windows. A bug submitted 7 years back for Firefox to handle system mailto calls, has been fixed. What this means is you can now set Firefox as default client for mail in Windows 10 and email links can be opened by Gmail in Mozilla browser if it has been registered within Firefox as mailto handler.
How to set Firefox as default Mail client in Windows 10
In Windows 10, Mail is used as default app for sending emails, if you want to make Firefox as the default mail client, you can, here is how.
1. Ensure you’re using latest Firefox Nightly.
2. Open Settings > Apps > Default Apps,
3. Click Mail under Email,
4. Chose and ‘Firefox Nightly’,

5. Done.

Now if you want mailto links to open by your favorite email service like Gmail in Firefox browser, follow the below steps.
Make Firefox open Gmail for email links
- Click on Hamburger menu
- Select Options > General > Applications
- Look for mailto, click to select it and choose ‘Use Gmail’.

Warning: Do not select Firefox above instead of Gmail in the third step, by doing so, you’ll end up Firefox opening multiple number of tabs when you click an email link, so avoid doing that.
Using Chrome? Read Make Chrome to use Gmail for mailto: links on Windows
Do you appreciate this move from Mozilla? Let us know in the comments below.