Comment by lucb1e
Sadly they dropped XUL some years ago. I stuck to that version for all the customizations it allowed, but became untenable and it was clear I couldn't run a browser from 2017 for the next 50 years so I bit the bullet. I'm now also on webextensions instead of real add-ons
Yet it's still more customizable for users than Chrome/ium is. That there is a particular customization they got rid of is a shame and what you mention in the sibling comment (only works in some contexts) bothers me every day when I try to use mouse gestures on a settings page or mozilla domain and it refuses to work, but those new limitations don't make the statement untrue as a whole
Firefox is less customizable than Chromium-based Vivaldi, so the statement is still not true