A viable alternative to the Blink and WebKit dominance to allow something other than every browser being Chromium.
Firefox’s Gecko engine is rather tied into the browser meaning nearly all Gecko based browsers are just Firefox with pre-config and extensions.
I’m keeping an eye on Servo and Vox as what seem to be the most viable alternatives currently in development.
I know it doesn’t tick the boxes but there technicality is a Microsoft made, open source, alternative to Explorer. The original File Manager for Windows 3.1 and it is still seeing active development. Just thought to bring it to attention for a bit of nostalgic fun but I actually find myself using it a fair bit.