>The WebExtension API even years later is afaik still less capable than XUL.
XUL extensions were an ungodly mess and arguably hurt Firefox more than they helped. Stability, performance, etc. issues were common to be caused by them. Which when everything is in a single execution context is not fun. Most of the popular extensions simply hijacked random internal Firefox functions which is why they couldn't just be installed together.
edit: I wrote the first Tag Groups extension back in the XUL days so I contributed to the mess.
I also wrote extensions and I did not find them to be an "ungodly mess". XUL was rather pleasant to work with compared to some of the other technologies I was programming with at the time
They were an ungodly mess, in my view, compared to modern approaches which is what matters in the context of this discussion. The issue also wasn't XUL but everything else related to how extensions functioned (same JS context as literally everything else, no processes, internal FF methods were being patched by extensions, etc.). Their power came at a great cost and their approach is inherently infeasible in today's security conscious world (even if you ignore the stability and performance issues).
XUL extensions were an ungodly mess and arguably hurt Firefox more than they helped. Stability, performance, etc. issues were common to be caused by them. Which when everything is in a single execution context is not fun. Most of the popular extensions simply hijacked random internal Firefox functions which is why they couldn't just be installed together.
edit: I wrote the first Tag Groups extension back in the XUL days so I contributed to the mess.