Hi, thanks for your feedback :-)
Sorry for my late reply.
I didn't have a repository for this yet, but I've created one.
You can find it here:
https://github.com/devmni/Certificate_Trust

If you open the add-ons directory (Firefox -> about:profiles -> your profile -> extensions folder), you can add/delete trusted/untrusted (green icon) CAs in "/db/IncludedCACertificateReport.json." The downside is that you have to do this manually each time you want to add or remove a CA. An add-on UI would actually be useful for management purposes. There could then be "customer-friendly" a option implemented to mark some official CA's as "personally trusted" - as like you suggested. The add-ons root directory also contains a Python update script (ca-updater.py.txt) that retrieves the current list of trusted root CAs from Mozilla and converts it to JSON.

I have to be honest, though, that I actually work in a different development area (and am not a web developer). So please have mercy. I simply cleaned up some old Cerdicator code, added my own code, added some nicer icons, and then packaged the whole thing.

So, if you take a look at the repo: There may be some code comments in my native language (German). Since I originally only forked this for myself - I'll replace them with english comments soon.