[dikei]

Firefox used to be extremely extensible: there were some software based on Gecko and Xulrunner. However, they all died out after Chromium and Electron took over.

[CmdrKrool]

Xulrunner specifically, died or at least is not easily revived, because Mozilla killed XUL and XPCOM.

[eb0la]

There was even an Oreilly book about it: https://learning.oreilly.com/library/view/programming-firefo...