The Custom Buttons extension worked fine in Mypal 68 from the very first release until the previous version 68.14.4b. @feodor2 also has this extension available and can certainly test what's going on. In any case, he has made significant changes. The menu bar is also affected and no longer behaves as it did before. This may also be one of the reasons why some of my CSS stylesheets no longer work. Same applies to the Add-ons Manager.
P.S.: @feodor2 And FYI, I also tried xiaoxiaoflood's method you recently pointed to and installed the boostrapped version of Custom Buttons (this extension always comes in two different versions). In this case, the extension options appears again in the Add-ons Manager (no error in the Browser Console in terms of the boostrap.js file) but when installing custom buttons, none of them are available. Although they are listed under the entry Custom Buttons in the Add-ons Manager, they are simply not accessible and their tool bar icons missing in the Customize window. Same to the Custom Buttons menu in the main toolbar. As you may know, Custom Buttons is an extension that also runs in newer Firefox versions up to the very latest. BTW, all was tested by me in a clean, fresh profile.
If you need all the no longer working stuff, please let me know. I will then upload it for you.
P.S.2: Furthermore, new errors can be found in the Browser Console compared with your previous release, even when opening the about:preferences page. Since you haven't been around here since 27. October, and the bugs are plentiful and serious, I even jumped over my shadow and reported it to your issues on GitHub to make sure you were aware of the problems with your new release.