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
the 'based' does the heavy lifting there. I said Firefox (the browser) not Gecko (the engine). I'm sure you can customize everything in either browser (or engine) if you download the source and recompile it with the modifications you like, or even hex edit the binary, go wild. But I'm sure you know what is meant...
When a word has two meanings you can claim "you explicitly said it!!1" but that doesn't make me having actually meant the other thing that I didn't mean, particularly when the original sentence makes a singular meaning clear: nobody can build anything on Chrome, why'd Chromium then suddenly mean "Chromium and any code modification you can make to it" and not simply "Chromium the browser as it is"? "Chrome/ium" referred to the two Google browsers, not forks that modify the underlying code, like Vivaldi which obviously diverges and is its own thing. The comparison wasn't Firefox vs every possible browser built on Chromium. If you want to compare Firefox to Vivaldi specifically, sure, I'll trust that Vivaldi fulfills your special wish if that's what you're saying, but that's a different conversation
How do you customize the default keyboard shortcuts?