At the time, Chrome had Chrome Apps. If you recall, several XUL extension authors quit in disgust and started developing chrome apps. Google wasn't considered 'evil' at the time...
Firefox had a unique, special power which differentiated it and Mozilla chose to kill it. No wonder its browser share dropped like a stone.
Chrome Apps were in no way replacement for XUL. They could not change the behavior of the Chrome itself, they just opened their own window and did their thing there. Not really an extension, more of a separate app.
To talk on more specific terms, which XUL extension did you replace with Chrome App?
I finally ended up using alternate tools since even the replacement Chrome Apps were pale imitations of what Firefox offered. The good ones needed to use Nacl just to do basic stuff and still couldn't reach what XUL extensions natively supported.
But I changed my browser to Chrome since Firefox lost its value proposition. It had a differentiating factor of superior experience and features that it chose to deliberately abandon.
The removal of XUL extensions triggered a large outcry among developers and power users, but I don't think it was all that relevant if you look at the graph of Firefox's declining market share. That was driven far more by Chrome's increasing share.
Firefox had a unique, special power which differentiated it and Mozilla chose to kill it. No wonder its browser share dropped like a stone.