How to make Gmail open when clicking mailto: links in the same browser it is clicked? (Chrome/Firefox)

You can make Chrome open when you click a mailto: link:

Access Gmail, and click an icon of the shape of overlapped double diamonds, next to the star icon in the omni-box where the URL is displayed.

If you don't see it, try reloading the page.

Source, by mulogics on Web Applications Stack Exchange

If you somehow missed the opportunity and said 'no' after the diamond question, your chance to undo that is under chrome://settings/handlers, where gmail will be listed as handler-question-blocked.

There is an option for that in firefox.

Edit -> Preferences -> Applications -> mailto

or

Firefox menu -> Options -> Options -> Applications -> mailto

gnome-gmail lets you do exactly that.

This is what reads from the Description section in the control file of gnome-gmail package.

support for Gmail as the preferred email application in GNOME

To install gnome-gmail, run this command:

sudo apt-get install gnome-gmail

After installing, open gnome-gmail and adjust your settings.

However, if and when you click on the mailto: links, it will open a Gmail tab in the preferred browser instead of whichever browser you are currently in.