For those that haven't made the switch to Firefox 57 yet, I thought I'd share some helpful notes:
0. 57 is fast!! I'd say it feels 2X to 3X faster loading the average page. It really does live up to the hype. I should also mention that 56 was hot garbage and was about 10X slower than 55, so if you're on 56, switch to 57 right away. Maybe they slowed down 56 just to make 57 feel faster :-)
1. 57 breaks most(!) extensions/addons; before upgrading to 57, make sure any extensions you depend on have been ported to the new "WebExtension" format; any non-WebExtension will not work at all.
2. I need NoScript and ABP, and both are now (as of a few days ago) supported in 57. Both have new interfaces, and NS seems like a complete rewrite. Both seem to work fine. BUT... the Fedora rpms I usually use for them have not been updated to the new (57-compatible) versions yet, so I had to remove the rpms and install the newest versions via the in-browser tools: I will reverse this once Fedora releases the appropriate versions. YMMV depending on your choice of extensions and whether you like the distro vs in-browser method.
3. 57 changes your profile in an irreversible way so you cannot revert back to <57 if you want without deleting your profile. Best to run cp -a ~/.mozilla /some/backup/dir before upgrading.
4. From what I've read, the latest Android FF does not use any "Quantum" tech yet, so everything I've written here only applies to desktop. I tested (only) on Linux.
P.S. If anyone else has problems with formfill dropdowns appearing over form fields in FF 54-57, please add your voice to https://bugzilla.mozilla.org/show_bug.cgi?id=1420356