Servo is being worked on as an alternative (after Mozilla excised it in one of the dumbest decisions they could've done), with the devs recently starting work on adding browser chrome, but it'll probably take years before it can be considered to be a useful alternative.
For now your best bet is probably to use a Firefox fork (Icecat will probably not shit the bed, but there's many other forks although a good chunk of them also modify the source code in ways that might not be desirable to casual browser users since they started usually with the goal to privacy harden Firefox and privacy focused forks have a bad habit of disabling features you probably do want enabled) or a user.js for Firefox to turn off undesirable features, at least until the situation becomes truly unusable; not sure what the best choice is after that because none of the Firefox forks have the manpower to step up and become a new main browser.
For now your best bet is probably to use a Firefox fork (Icecat will probably not shit the bed, but there's many other forks although a good chunk of them also modify the source code in ways that might not be desirable to casual browser users since they started usually with the goal to privacy harden Firefox and privacy focused forks have a bad habit of disabling features you probably do want enabled) or a user.js for Firefox to turn off undesirable features, at least until the situation becomes truly unusable; not sure what the best choice is after that because none of the Firefox forks have the manpower to step up and become a new main browser.