Hi samchristo, I don't have Windows 10 handy to experiment, but if you return to the Windows Settings > System > Default apps, you should find these options at the bottom of the list on the right side:

• "Choose default apps by protocol" -- this might allow you to assign Firefox to handle the mailto protocol
• "Set defaults by app" -- this might allow you to assign the mailto protocol to Firefox

If Send To doesn't use the mailto protocol, hmm, it probably has its own registry setting (shell handler). Would need to get hands on or get luckier finding the steps online.

The only option available in the MAILTO protocol is Windows Mail. I'm trying to figure out how to get Firefox/Gmail into this list. Thanks for your reply.

If you approach it from "Set defaults by app" does it let you associate mailto with Firefox? Or does it not recognize Firefox as an app?

Firefox is not considered an app and does not appear in the list. Microsoft, apparently, has hardwired Mail as the solution.