I think a lot of people project some war between browser vendors that doesnt reflect reality. Only speaking for myself (as a Mozilla employee) but I want what I believe to be the best for the web, not for Gecko / Mozilla (luckily Mozillas entire goal is to do the best for the web, so we have joint interests)
Since "Mozilla is a proudly non-profit organization dedicated to keeping the power of the Web in people’s hands" then it is able to do things differently.
However, users are basically self-interested and short sighted (basically "how fast does a tab load") so just being morally superior isn't a win ;-)
Of course, it does come down to individuals in the end, and
open source has pointless turf-wars, so nothing is guaranteed....
If just Opera switched to it, that would be bad - loss of one implementation is almost always bad - but it would at least maintain some non-WebKit market share in mobile. So less bad but still quite bad.
So yes, I am guessing that Mozilla folks would say it would be negative (I would).