Does anyone know why Microsoft chose to partner with their arch rival and not with Mozilla? Even if Chromium is a bit easier to put a new skin around, that's a

Does anyone know why Microsoft chose to partner with their arch rival and not with Mozilla? Even if Chromium is a bit easier to put a new skin around, that's a few developer years, nothing for MS. But strategically, handing it all over to Google, just like they did in the mobile OS space, what? How does that make any sense? Especially now that Firefox became an _excellent_ browser again?