Two reasons why I think it's unwise to move from Firefox.

First, browser engine diversity matters. If all browsers become chromium skins, this gives Google a very unfortunate amount of power. It can affect web standards, de facto web use, all kinds of things; and there's no guarantee chromium will keep being freely released.

Second, pretty much any other possible alternative is worse across a set of metrics that involve accessibility, access to source code, privacy, and safety.

Servo is promising, but a11y isn't there; so it may as well not exist for me.