Well, the speed improvements mainly come from GPU offloading and multithreading, so there's not much in it for us because the capabilities of our GPU are negligible and with a single core CPU there's no point in multithreading.

During the past two days I tried to backport the Debian Sid package of Firefox 57 to Jessie for Easy Debian, but there are a lot of obstacles along the way:
- a bunch of libs: easy
- clang: solved
- rust, cargo: probably solved
- slight incompatibilities in FF's python build scripts with Jessie: arguably handled
- FF dynamically changing its source code during the build process: maybe managable with tenacity

I finally gave up when I concluded that I would most likely need to backport g++ 6, which would be a hell of a lot of work, assuming that it would work at all. I highly doubt that it would, because it basically has the same prerequesites as glibc, whose incompatibility with the old Maemo kernel is the reason why I can't upgrade Easy Debian beyond Jessie.

I'm not saying that FF 57 is impossible to port to the N900 (with or without an inter layer like ED), but it's certainly not withing my skill set of mainly backporting Debian packages.