The issue with Firefox isn’t so much its visual design (though it is a component) as it is things like interactions all feeling a bit “off” and little bits of lag scattered throughout (like frames for controls and windows being blank for a split second before drawing in). It gives a “made in my back yard” impression, and not in the good handcrafted way but rather the duct tape and bubblegum way.

I don’t think Mozilla is going to be able to escape this so long as the they continue to use their current UI stack.