Not a good thing if Mozilla wanted to increase their market share.
Middle-aged users are already using Firefox if they value the open web, but young users will be driven away by the 2000-ish UI. Mozilla cannot attract new users if they can't keep up with the design trend.
The best solution I can think of is making fancy UI the default but also making it possible for power users to switch back to the old UI.
Genuinely interested: do you have any numbers backing those claims about UI, or is it your intuition?
My intuition is that the vast majority of people use the first browser they are exposed to, even if it is IE6. People are exposed to Google Chrome first, it's not a question of UI/UX.
Otherwise, why would the vast majority of people use Google Chrome instead of one of the gazillion Chromium-based clones? The UX is the same, right?
Middle-aged users are already using Firefox if they value the open web, but young users will be driven away by the 2000-ish UI. Mozilla cannot attract new users if they can't keep up with the design trend.
The best solution I can think of is making fancy UI the default but also making it possible for power users to switch back to the old UI.