Not even Brave, founded by Mozilla's ex-CTO/CEO forked Firefox. Microsoft Edge switched from homegrown to Chromium.
These two clues should be enough to cement your idea about how hard it is.
Even more important though is that it doesn't matter. You don't have reach. Apple locks you into Safari on mobile and Google pushes Chrome on Android and services having billions of users, like Youtube.
That's Mozilla's real problem. They lack in engineering power but engineering isn't the problem anyway. It's reach.
I think it's less about being hard and more about it being expensive redundent work. You will have to spend a lot of money to essentially make a duplicate version of an existing piece of software which you can use and build upon right now.
True, and if we're honest, the result of this hard work is a negative. Nobody in commercial software development benefits from having multiple slightly different engines. It just means more testing and more bugs, differences in feature support, the like.
These two clues should be enough to cement your idea about how hard it is.
Even more important though is that it doesn't matter. You don't have reach. Apple locks you into Safari on mobile and Google pushes Chrome on Android and services having billions of users, like Youtube.
That's Mozilla's real problem. They lack in engineering power but engineering isn't the problem anyway. It's reach.