I don't think they're unfounded. Nearly every developer I know uses chrome, and chrome dev tools. This wasn't always the case. Market share can be a lagging indicator, i think it's a little short sighted to suggest they're okay. They're not!
Forking isn't a sin. Diversity often a result of forking. Take a look at this graphic[1]. There's a good chance the browser you're currently using is a result of a fork.
Mozilla has to really examine what they stand for. The open web doesn't depend on implementation internals. Just specs, tests, and sensible governing body, and a set of willing participants that don't have unreasonable misaligned interests. They can still promote an open. Perhaps even do a better job of it with the extra resources.
[1] http://upload.wikimedia.org/wikipedia/commons/7/74/Timeline_...