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.
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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.