Conkeror will not start with FF 57 (nightly and beta at least) and with an existing profile won't print
any errors to console. I
found that if I created a new profile (with FF57) then it would print errors.

There are at least two major JS language changes in FF 57 that need to be fixed in conkeror before it can
start. They are the deprecation of "for each in" loop and requiring of functions that will be yielded
from being marked with function*.

After those are fixed, then there are at least several (maybe many) interfaces and APIs that have been
removed. Whether there are replacements that we can use is TBD.  This aspect, and expected changes in the
future, appear to be an insurmountable challenge to Conkeror's supporting FF >= 57.

With the commits on this tree https://github.com/scottjad/conkeror/tree/sj-ff57 I was able to get a
Conkeror window starting. Keybindings do not work (should be fixable), I think because of an error in the
first commit (for each in loops) on that tree not because of FF 57 API changes. Because of the large
number of yield* changes that were required to even get Conkeror starting, I'm sharing this WIP in case
someone is very motivated to work on this issue.