I was really excited for Quantum, but it appears the switch to WebExtensions is going to gut most of my favourite addons, and completely kill the rest. About half of the addons I use have been updated to be compatible with Firefox 57, so the rest will stop working as soon as I update.
Of the ones that have updated, most are missing significant features. The most significant example is Lastpass, which can no longer fill in http authentication boxes, and can't even copy usernames and passwords to the clipboard anymore. (The Chrome version has never been able to do the former, but can at least do the latter.) Lastpass fills many auth modals for me every day, so losing that feature is going to be a huge annoyance.
NzbFox is gone. Hilarious webcomic manager gone. Foxyproxy missing features. This just sucks. I understand the logic behind this move, but addons were one of Firefox's strengths. Now it appears they will be a sort of poor cousin to their Chrome counterparts. I would happily sacrifice speed to not completely hamstring the capabilities of addons.
Obviously it's too late for this, but the could have added the WebExtension system to provide greater stability, security, and developer ease for new addons (and those that wished to migrate), while maintaining the more powerful system for major things that required those capabilities. Yes, in theory it means a bad addon could compromise security, but that's true of any program I choose to install on my computer. I certainly wouldn't trade my general purpose PC for some locked down app store box regardless!