Listen, you're doing it all wrong. You're supposed to fork it, tell noone, do a years worth of work in secrecy, release it. palm it off to another FOSS community, then abandon it, then refork it, do another 2 years of work in secrecy and then release it again under a new name. Got it?
You'll never get to play alongside big boys like Apple and Facebook with this 'talk to upstream' attitude of yours. That's just not how the game is played.
If their only plans were to add that one feature I'm sure they would have done just that. Clearly they have grander plans than simply integrating existing functionality, and they felt they needed total control of the project to do so.
The beauty of open source is that if the maintainers of libjpeg-turbo want to incorporate it, they can, and I'm sure the Mozilla devs would be more than happy to help.
Edit: A good reason given in the reply by joshmoz below.