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).