Crap. I guess this means no more hacking the extension's .rdf files to raise the minimum version level so old extensions will work on newer versions.
I have about 40 extensions I use and without them, FF is useless to me. I may have to just dump it and move completely to Pale Moon.
Yeah, hacking extension .rdf files won't work anymore. See the below link for add-ons that are ready for FF 57. You can do a search to see if any might replace what you will lose. There should be more in upcoming weeks. Unfortunately, mine (about a dozen) are still labeled legacy.
https://addons.mozilla.org/en-US/firefox/search/?tag=firefox57&page=