Install Classic Theme Restorer if you miss your overly complicated UI ...
The recent version of Firefox did not remove the complications, it removed the ability to customize.
If Mozilla wants to have a simple, dumbed-down interface for Firefox as the default, I am absolutely OK with that PROVIDED that Mozilla retains the ability in Firefox to customize the interface to the complexity level that I want.
That is the problem with Mozilla's current thinking. Mozilla has removed the ability to customize Firefox. The "Classic Theme Restorer" is a bug-fest. It's the equivalent of putting a pile of shit out in a fly farm.
if Mozilla strategically intends to have plug-ins or add-ons provide essential functionality of a browser, then Mozilla needs to implement a quality assurance certification program for those plug-ins and add-ons. The current "works for me" ratings systems fails miserably in this regard.