Yesterday, the Mozilla people released Firefox version 57. And with it, they have removed the old "XUL" based add-on system. So all add-ons must now use the new WebExtension API. Unfortunately, many add-ons (including a lot that I use) have not been updated yet, despite Firefox issuing warnings for the last several revisions. In many cases, the authors have abandoned the code, and in others, I don't think they care enough to want to re-implement it for the new API.
The result is that I've had to remove and replace many of my favorite add-ons. Here's a list of what I had to change and what I think of the results.