How about: Mozilla HTTPS To My Router (or printer or any other physically present local object) in a way that does not utterly suck?

Seriously, there’s a major security and usability problem, it affects individual users and corporations, and neither Google nor Apple nor Microsoft shows the slightest inclination to do anything about it, and Mozilla controls a browser that could add a nice solution. I bet one could even find a creative solution that encourages vendors, inoffensively, to pay Mozilla a bit of money to solve this problem for them.

Also:

> Thunderbird for iOS - why is this not a thing yet?

Indeed. Apple’s mail app is so amazingly bad that there’s plenty of opportunity here.