I have tried to make it as similar as possible to BBCodeXtra, which provides a context menu that automates many bbCode/vBulletin/MarkDown/HTML markup tags.
What is not done yet is the documentation and the localizations.
The menu structure is otherwise identical, though the custom tags and configuration is different because of the differences between XUL and Web Extensions.
Also, while similar, the syntax of custom tags is a bit different.
I have it up at GitHub, it's called bbCodeWebex
You can check it out by using the temporary load option for FireFox using about:debugging
Updates as they occur.