I'm sure you heard of it by now, but just in case.
With Firefox version 57; all current addons will stop working, and any that can't be ported to the new, much less powerful, extension framework will be gone until the framework is improved or hacks are found. They're also going to massively fuck with the GUI no reason(again.)
Many forks are out there, such as iceweasel, waterfox and pale moon. Some of these are likely to stay on the current system; Pale Moon certainly will. However these projects are smaller(Pale Moon is basically one person) and are unlikely to get Grade A security updates, and they won't be able to keep the current Firefox addon development community from shattering into a million pieces. Look into these now, be prepared.
MAKE BACKUPS of the .xpi files of your favorite addons. I don't necessarily see Mozilla taking down the old versions immediately after they make the switch, but it's unlikely they'll keep them around for posterity when the only version they care about can't use them!
This is coming before the end of the year, and Mozilla thinks anybody who doesn't like it is just confused or something. We're on our own. Netscape is dead. Long live Netscape.