This.

Plus worth mentioning that Firefox has native support for real user stylesheets with own origin layer, just as specified by standards. It is userConent.css file in the "chrome" folder in the profile directory. Slightly cumbersome compared to Stylus' provided "UserCSS" injections, but I think it is super important for any user agent to keep this functionality that is by the way required for CSS2.1+ compliance, and UAAG [1]

And also, to my knowledge, Firefox is one of few browsers that lets you (still) hack its interface ("chrome") through css (userChrome.css in aforementioned folder). [2]

[1] https://www.w3.org/TR/2015/NOTE-UAAG20-Reference-20151215/#s... [2] https://twitter.com/myfonj/status/1387584962354982912

Container tabs for me is the killer app. being able to stop google work creeping into my personal youtube is really useful for me

Also bookmarklets!!! I use bookmarklets with keywords to put a ton of functionality into my URL bar, it's probably my favorite part of Firefox

>I can customize pretty much everything about the browser UI using CSS.

Not the controls, though. For that you’d need Firefox 2016.

Which controls do you want to remove? Can't you do that through the customize toolbar? I'm removing the toolbar and the sidebar header with the following userChrome:

  /* to hide the native tabs */
  #TabsToolbar {
      visibility: collapse;
  }

  /* to hide the sidebar header */
  #sidebar-header {
      visibility: collapse;
  }

I didn't ask to remove controls. I referred to customizing controls to the level you could as of Firefox 2016. Or like you could do in Super Metroid (1992). For example, if I want shift-J to move to the left tab, and shift+K to move to the right.

As of now, extensions can kind of customize this like before, they just don't do it in a fully general way (like you'd get from a supported settings feature). The extensions will only apply after a given tab's page has loaded. So if I want to go left three tabs, I will get stuck on any tab which needs to reload, or doesn't have a page loaded, or any number of exception.

So, bottom line, I can't customize controls in a general sense anymore.

I agree it is a nuisance and I also am looking back to Keyconfig days with ton of nostalgia, but I've learnt to somehow overcome this in general with third party key remapping, in my case pretty basic AutoHotkey directives that simply translates "my" key combinations to "theirs" keyboard shortcuts. Can be possibly "scoped" only for given app, yet I have ended up leaving these mappings global, since they mostly make sense for any application in general.

For example I like to be able to have two distinct "close tab and focus left" and "..right" shortcuts, so I set every app to ditch MRU switching of tabs and always "close to right" (Ctrl+W, not remapped) and then have other (Ctrl+Shift+W) combination set to fire up Ctrl+Shift+PgUp (what moves active tab to the left in most apps) and then Ctrl+W what closes it natively, so new active tab is the closed one's former left neighbour.

    ; Firefox "close tab to the left"
    ; relies on about:config browser.tabs.selectOwnerOnClose=false
    ; (that ensures ^w will always select → RIGHT
    ; #IfWinActive, ahk_class MozillaWindowClass
    ; close to left
    ; this will effectively override the dreaded ctrl+shift+w close window
    ^+q::
    ^+w::
    sendInput,^+{pgUp} ; = swap current tab with left neighbour
    sleep,5
    sendInput,^w ; = close current tab
    return
    ; close to right (default)
    ;^w::
    ;sendInput,^w ; = close current tab
    return
    ; #IfWinActive

The same shenanigan is doing (also global) mouse gesture for me (StrokesPlus, the olden one).

In addition to all of the reasons so many have mentioned, my favorite reason is this setting which allows me to easily use my preferred fonts on all websites: Allow pages to choose their own fonts, instead of your selections above

De-selecting the box and specifying my preferred fonts is the absolute first setting I change on every Firefox installation.