"Servo is a successor to Firefox" isn't really accurate. It's a research browser engine, not a production browser, and there are no productization plans at this time. (That said, we're aiming for the same engineering quality of a product—"research" isn't an excuse to cut corners.)
It might not be a goal for Mozilla for it to be a production browser but it does seem to be for other people working on it and this is affecting what is being worked on:
I'd like to point out the significant between "browser engine" (HTML/CSS/rendering, eg WebKit) and "browser", which relies on a rendering engine but encompasses a lot more functionalities. Servo, as far as I know, is the former, presenting it as a "browser" (even if only at an experimental level) is misleading.
"Servo is a successor to Firefox" isn't really accurate. It's a research browser engine, not a production browser, and there are no productization plans at this time. (That said, we're aiming for the same engineering quality of a product—"research" isn't an excuse to cut corners.)