It used to be EXTREMELY personalized and personalizable, right out of the box, even without extensions. Mozilla has systematically REMOVED FUNCTIONALITY from the browser, and I'm not talking about switching from XUL. Personally I don't care if XUL ever comes back. I don't even use many extensions, mostly just uBlock.
A couple of versions ago they took out right-click for image properties! a basic function of browsing since Netscape Navigator and didn't add it back until two more releases. They took away my compact view mode. They foisted less powerful menus on me and removed configurable settings from about:config, the single most powerful customization feature of the core browser.
They keep chasing a new Chrome-like feature or look while killing things I use regularly. I don't want that shit. I want the old customizable feature-rich Firefox experience I had back in the 2.0 days.
So many users eject after each interface overhaul or silent removal of basic functionality and Firefox's trademark customizability. Yet they kept doing it. Madness.
This.
I used Firefox since back when it was known as Phoenix. It was my primary browser for many years, but I ended up dropping it back when they launched the "Australis" makeover. I didn't like the look of it as it was too similar to Chrome, but my biggest beef was just how much customisability they removed. That's been one of my constant criticisms ever since.
Firefox's USP used to be the flexibility and customisability that it had. Sadly, Mozilla seems to be determined to remove as much of this as possible, and hence for me one of the biggest reasons to use it has been steadily eroded. These days I typically use Pale Moon for most things (a Firefox fork that retains much of the customisability Mozilla removed), with Vivaldi being used for certain sites that don't play well with Pale Moon.
From time to time, I do try Firefox again, but it just doesn't have enough to tempt me back. Maybe this may change? I hope so as the web does need none-Chromium browsers.