Yes, dev tools features could be expanded as well as the plugin franework(vimperator!).But these are non-issues for regular users.
XUL extensions removed, power users are angry and leaving. And the power users were the main driving force for Firefox adoption - mostly not by themselves, but as influencers for people around them.
What an irony indeed.
For example, I was unable to use NoScript, because it didn't really work well.
Still, this is something you can forgive considering that for more than 15 years they released always good software.
It was a slap in the face of the core user base and they did it carelessly. They did not have the necessary information in place early enough and did not fix bugs that prevented people from porting their addons before the end of the last pre quantum ESR. They more then deserve where they are today
Man, a web browser that works on all the current existing devices on the planet is not something that I would consider trivial, at all.
Internet Explorer needed a brand new operating system that forces it for anything you do to regain some momentum...
Apple has to offer obviously stuff that you can't do easily with other browsers, like integration with the password manager, etc...
Google Chrome... We will see what happens in 10 years. They are making more and evident what they developed that browser for.