Mozilla redesigned the browser 4 years ago. At that time, I was expecting this change for the best, making Firefox as advanced as Chrome, if not more and more intuitive while still retaining the simplicity of UI, is what I thought.
4 years on and I still can’t make myself go back to FF. I miss the old simple UI. You could choose any extension from the extension store and personally, the best part was theming which is all gone! The browser is now all clustered. And Mozilla is still using the old screenshots of the browser in Play Store, I wonder why.
You can still more or less do this if you create an add-on collection and add all your extension to it. In order to register this collection with Firefox you then have to enable the Debug Mode by clicking the firefox logo in firefox’s about section 5 times.
It may be possible that you need Firefox Beta or Fennec for this to work
You currently need fennec or firefox nightly or whatever, but they just recently announced they’re working to move it to regular firefox also