I recently switched from Chrome to Firefox, and was surprised to see that
there's no easy way to open mailto links
with Google Inbox. I searched online and found a
bunch of forum posts about this issue, but I didn't find any answers. Nothing I
changed in about:config seemed to do anything.
After much frustration, I found the Firefox protocol handlers documentation and got something working. Navigate to Google Inbox and open up a Javascript debugging console (Ctrl-Shift-I). Then type or paste the following code into the console:
navigator.registerProtocolHandler("mailto", "https://inbox.google.com/?mailto=%s", "Inbox");
Once you do this, you should see a notification appear at the top of the page asking if you'd like to use Inbox for mailto links. Click yes, and you should be good to go. You should be able to use this same technique for other webmail providers (ProtonMail, etc.).
For your convenience, here's a mailto link you can use for testing: evan@eklitzke.org. Feel free to send me an email telling me how thankful you are.