When will Microsoft start working to remove their arbitrary limitations on Windows Phone so that Google and Mozilla can port their browsers to it?
http://code.google.com/p/chromium/issues/detail?id=153802
The other stumbling block was a licensing issue where the Windows Phone Marketplace prohibits copyleft licenses, which prevents software like Firefox from being published there:
http://arstechnica.com/information-technology/2011/02/window...
That's again from the WP7 days, so the agreement might have changed since then, but if it hasn't, and if there's no way to sideload apps onto WP8, then it's a major blocker for Firefox.
Opera's last statement on the matter was on Oct 1, and at that time they were still waiting to see what avenues the SDK would allow.