There is clearly a lot of effort fractured over a lot of projects, perhaps enough to actually take on the task, but I'm not aware of any project committed to a hard fork of firefox, with everything that entails (and if you had the resources to go that route, I'm not convinced that is the best path to take either)
I can only really think of a handful of groups I think have a chance at success, but I don't think any of them are currently considering it.