Update from original poster (I don't see any way to edit or update my own posts in this forum):
Well, I've learned a few things ...
1) I've no idea how to convert the existing add-on to the new format, and there's a *lot* to learn before I'd be able to 🙂 . Not something I'm going to be able to just do in a night or several; I'd need to learn all about how the old style of add-in works first, in fact.
2) I have, however delved into the existing code and found that ColorfulTabs stores fixed colors for tabs in sessionstore.js . Its entries, part of other stored tab info, will look something like this:
---
"extData":{"tabClr":"238,238,238"}
---
It could also have other attributes in the "extData".
3) By inspecting sessionstore.js before, during, and after running Firefox and setting tab colors in the program and altering values of the "tabClr", I have determined that -- for whatever reason -- ColorfulTabs is not storing the changed values after I have changed the color of a tab and closed the program, and it is not reading the values from sessionstore.js and implementing them when I have manually changed them in the file while Firefox is not running.