In November 2017, the Firefox Browser has been upgraded to Firefox Quantum. The code base has been significantly rewritten and, as a result, the existing add-ins are no longer supported. Unfortunately, this also means that the Reference Resolver toolbar for Firefox is also no longer functional. My sincerest apologies to all Resolver users on Firefox.
I am extremely busy at the moment, working in the lab and trying to finish my two papers before Christmas. This means I will not be able to write an updated Firefox add-in until I publish (Christmas break?) The API has changed significantly and it will take several days of careful reading and writing the new version essentially from scratch.
In the meantime, you can opt to stay with Firefox 56, the last "old style" browser version. Or you can switch to Firefox Extended Support Release – the version with earlier engine but with all latest security patches.
If you have any comments or suggestions, feel free to drop me a line.