The date bomb for this add-on is now in effect, and the add-on will no longer function in Firefox, unless you either roll back your system date, or, the instructions here may help you: http://tl.gd/n_1sqcu4r

Again, to the developer: There are XUL variants of Mozilla that people are now using, such as Waterfox and Basilisk. Please make a separate XUL based version of your legacy plugin for those users, and separate out this crippled version for Web Extension only versions of Mozilla (i.e., Firefox 57+). And please remove the date bombs, there are not the slightest bit necessary or user friendly for that matter.