> The XUL extension mechanism was permanent technical debt loaded onto the browser because of how it exposed features, basically welding things directly onto the browser's guts, which is a nightmare to maintain.
There is no evidence for this at all. Extensions can't modify the rendering engine.
"guts" meant the XUL implementing the Firefox UI. tialaramex is absolutely right about that, extensions had total access to that XUL/JS state, which is why changes to the Firefox UI inevitably broke extensions.
There is no evidence for this at all. Extensions can't modify the rendering engine.