As a Rust dev, and nerd generally, I love the idea of servo and if I were more excited about browsers and the web would even love to contribute to it. But unfortunately stuff like this isn't what makes or breaks project success in the short or even medium term.
I use Firefox as my main browser everywhere now, but I didn't make the jump on any technical merits in rendering. Most days I barely notice which browser I'm on. I use Firefox because of the principles it represents and Google finally crossed the final lines with me, ethically.
I don't know what the future holds for Firefox and Mozilla but I don't think it's fundamentally strongly altered by the success of servo.
In any case, as a software engineering community we need to find ways to fund big picture projects like Servo etc. without having them tied to the fortunes and whims of various corporations. We lucked out with Linux filling a niche where it could explode in popularity and be securely funded for dev, but that hasn't been the case with lots of other things.