1) Changing your UI to be more similar to a worse UI is fucking up that UI. The UI includes far more than the shape of the buttons or the complete disregard for a well designed scroll bar. One of the things they intentionally made worse in order to improve bullshit metrics of their Download toolbar button was the "Save File" vs "Open File" feature. Previously the "Open File" created a temp file that disappeared when you were done with it. This was really useful, such as when opening a bank statement then saving it with the proper statement date (which isn't included in the filename of most statements you download). The updated version of that saves the file in your Downloads folder and you have to manually remember to delete it. Firefox viewed the extra clicks on their toolbar button (Click -> Right click on file -> Delete) as improved user interaction (and thus claimed people enjoyed that feature more) rather than the annoying, wasteful extra steps it actually is. Intentionally making workflows worse is fucking up the UI. They do it a lot and just because other companies do it too doesn't give them an excuse to continue doing it. It's possible to be better than everyone else and not fuck things up. They intentional choose not to.
2) They absolutely did fuck up the extensions. The extensions that are important to you aren't the same extensions that are important to other people. Yes the browser needed to be updated, but they didn't release new APIs that covered all the possibilities of the older extensions. There were a lot of UI things the old extensions could do that they no longer can. (Also the UI for just getting to an extension's settings is horrible.) Though I agree with you that this is an old issue. But still, they could be improving the extension API to give more control over the browser but it doesn't seem like they are.
3) One of the original points of Firefox was that features were extensions. That's completely gone. All these new features should have been provided as extensions. Further, the settings to turn things off are very certainly not clearly documented. about:config could have a description column but doesn't. You can't consider anything in there as documented. The Settings page is very poorly designed and doesn't let you toggle a bunch of things on/off. None of the settings have tooltips and only a few have additional help/descriptions linked with them. The page is poorly designed because there's a huge mixture of UI controls with some changing the current page, others opening pop-ups, others opening in new tabs, hiding/showing new options, etc... and it's not clear which of those controls will do what. Some block you from going back, or at least some give you a Back arrow and others don't. There's settings grouped by sidebar 'tabs' but there are groups within those tabs which don't have indexes nor give you an overview of where something is. Sometimes there are lines separating some of those inner groups and sometimes not. There's a mishmash of different text stylings. If you study it closely you can figure out there's subgroups of subgroups, but since nothing is indented and there's no index, it just looks like a mess.
4) Yeah, there's a ton of idiots who say they don't like change X so they'll switch to a browser which has even more things they claim to dislike.