How do I stop Gmail from opening mailto links in Chrome?

Solution 1:

The mailto function opens the primary email client. If you want to use Gmail as your client there is no way to open mailto in Thunderbird.

If you want to move from Gmail to Thunderbird you should try to set it through the Thunderbird preferences. There should be a button like 'check if Thunderbird is default email client'.
As an alternative follow Ali's description.

Solution 2:

1. Launch the default Mac OS X email application (called "Mail", located in the /Applications folder.)
2. Select Mail | Preferences from the menu.
3. On the General tab, select "Thunderbird" from the Default email reader drop-down list.
4. Close Mail.

Solution 3:

Go into the Chrome browser settings, then scroll down to "Show advanced settings". Look for "Privacy" and then click on "Content Settings". Scroll down to "Handlers" and click on "Manage handlers". You will see your "mailto" handler there and you can deselect Gmail. Hope this helps!