Practically, it's impossible to maintain one without a proportionally large team that can spend proportionally big amount of time.
As one of most obvious examples (unrelated to Mozilla or Microsoft): there are uncountably many custom Android ROMs out there, left abandoned. And most were merely cosmetic changes or even packaging (like inclusion or removal of certain pieces by default). Usually, unless luck is that there's a large social momentum and a lot of advertising among same-minded people, fork maintainer struggles for a few versions then gives up and surrenders to the upstream way of thinking. Sad but true.
Forking usually happens when a group or individual is unable to steer the direction a project. Having to go on your own is pretty much the opposite of "having a say".
