You are making a false apple to orange comparison, i.e. comparing Edge without any extensions to FireFox with extensions.
The new version of Edge will support not only MS extensions but Chrome based extensions also. In my experience, there are more extensions and addons for Chrome based browsers than there are for FireFox.
Wrong: Chrome based extensions come far behind in the number and breadth of functionality that legacy Firefox accumulated over the years. In totality it cannot even match the functionality that Tab Mix Plus provided ( PC World said that "With Tab Mix Plus, Firefox tabs go past the obvious and into the indispensable... it's hard to imagine how you lived without it.") along with Colorful Tabs. Show me the Chrome extension than enables multiple tab rows, or adjusts max or min tab width, or even a simple option like enabling Ctrl+Tab to toggle btwn more recently opened tabs, rather than the ridiculous through all tabs (there is an extension that is sppsd to do this but apparently it does not do that, but sets up alternative keys, and I have not figured it out yet). Or adds Copy Url on the right click menu for a inactive tab. Or that keeps the Find bar open across all tabs, Or any of the other functions I listed (and there are more), with the possible exception of the Open With extension I found.