It's not about Firefox-only vs standard, but about control.
When your extension does not or cannot work anymore due to lockout by browser consortium agreement, the control isn't with the user or the developer -- it's with the browser vendor. It kind of defeats the essence of extensions in the first place, which started out as a technique to give users and developers power over their user agents, and that's being reversed.
If what your user agent can do is dictated entirely by a consensus of a small number of browser vendors, the web becomes less open. Neither users nor developers wanted this.
That's a very broad statement to make, and I don't think you're in a position to say something like that.
For any extension that can be written with WebEx, this move is unambiguously a good thing. The developer can now just write their extension once and deploy it to multiple browsers. And for their users this is also a good thing, not just because reducing maintenance burden on the developer is more likely to lead to faster development, but also because AIUI WebEx has a much better permissions model than the old extensions.
The only real problem is the subset of extensions that cannot be rewritten with WebEx. Losing these extensions sucks, to be sure, but it's a tradeoff. You just can't claim that this move is strictly negative.
When your extension does not or cannot work anymore due to lockout by browser consortium agreement, the control isn't with the user or the developer -- it's with the browser vendor. It kind of defeats the essence of extensions in the first place, which started out as a technique to give users and developers power over their user agents, and that's being reversed.
If what your user agent can do is dictated entirely by a consensus of a small number of browser vendors, the web becomes less open. Neither users nor developers wanted this.