... and almost all my extensions will cease to work. 16 legacy, 4 ready for 57. a few of them i use very often: automatic save folder, save file to, video downloadhelper, downthemall!, search by image for google, image zoom, copy links
You can switch to Firefox ESR to keep using your extensions and give the extension authors more time. ESR 52 will be supported with fixes until June. https://www.mozilla.org/en-US/firefox/organizations/faq/
The problem is that certain valuable extensions are impossible to update, because the new plugin API doesn't expose the necessary functionality. E.g. Keysnail, which will break because Mozilla won't expose the ability to override C-n.
Mozilla should have taken the time to release a fully-baked extension API before deleting the old API; then it really would have been on the extension authors. Between this and the utter destruction of Sync's security guarantees a few years back, the Cliqz keylogger fiasco & the Pocket fiasco, as well as the WebRTC fiasco (as well as the defenstration of Brendan Eich), I'm very disappointed in Mozilla these days.