They'd notice. Maybe they wouldn't notice that they notice, but they'd get a general feel that things look worse.

Agreed. I used panopticlick and it showed me that web fonts were one of the biggest discriminators. I disabled them and tested on a sample of normal end user type Web sites. The web got real ugly looking real fast. If Mozilla disabled web fonts by default, normal end users would draw the conclusion that Firefox couldn't properly render the web, and would flock back into the arms of Chrome.

They could ship web browser with all common web fonts to render them from disk without fetching. This would allow disabling of fetching of web fonts without breaking websites. Also could eliminate fonts as a fingerprinting vector altogether if system fonts were ignored as well.

What about fonts which are not licensed under an open source compatible license. What about fonts that are not "common" but are used by some popular Web site? Your request is doomed to failure. Like it or not, Web authors have been given the power by Web fonts to have total control of the fonts on their Web pages, and they ARE using that power. Any attempt by a low market share browser to change that fact is doomed to blow back on that browser.

I'm sure they can afford to buy rights to important non open source fonts or fund the design of similarly looking open source fonts to ship as aliases. There is no reason for the madness with fonts to continue.

That’s actually not a bad idea. Lots of sites do have custom fonts, so maybe it would be a good idea to still put some icon in the address bar when the site has web fonts that didn’t come with the browser.

Decentraleyes does exactly that (at least with scripts, I do not know if they do the same with webfonts and css files)

Even google news renders weird without custom fonts if I remember correctly. Lots of sites use font files as icon sets etc.

The only think that they would notice is when buttons break due to the lack of web fonts.