In order to conquer the internet yet again, Mozilla has decided to make rebuild the Firefox UI with modern web technologies as a replacement for the current XUL (XML User Interface Language).
The XUL has caused performance issues and developers avoided it, Firefox being the only browser to use it on a regular basis. Many developers decide not to create Firefox extensions because of this interface language.
Most likely, XUL will be replaced by a combination of HTML, CSS and JavaScript. Also, by dropping XUL (Mozilla specific language), the Gecko engine, which the browser is based on, will become lighter.
I am an old Firefox user myself (meh, I have switched to Pale Moon, but that is still Firefox) but I think they need to rethink the whole “browser business” and to bring unique features if they want to become the #1 Internet Browser again. Though competing with Google and winning is almost impossible.