They didn't sell the space to Booking.com. They've promoted one another in their user bases: Firefox showed a message about Booking, Booking showed a message about Firefox. There was no money involved.

I'm not gonna defend their action to do so because I disagree with it too, but if you wanna feel outraged about it, at least get the facts right. John Gruber should do the same.

A sale is a sale even if it doesn't involve hard currency. When Disney bought most of 21st Century Fox assets, there was no money involved either.

What privacy-related information was sold to booking.com again, regardless of how the transaction played out?

I get the feeling that people are asking Mozilla to simply not deal with corporations at all, regardless of privacy, which is simply impossible if it wants to survive - almost all major actors are corporations, so the alternative is Mozilla turning into a shut-in.

If you want influence, you're going to need to deal. And sure, there are deals conceivable that represent giving up on principles for short term gain. But just because it's possible to conceive of a deal that's "selling out", doesn't mean every deal represents selling out.

I mean in this very same comment thread another poster is complaining how principled Mozilla is @ the w3c.

Suffocating Mozilla (regardless of whether you use/admire/detest/ignore Firefox) under impossible expectations is likely a permanent loss for users. There are precious few organizations like it with any influence that are advocating for the individual in web tech, so once Mozilla dies (which seems likely, at this point), whatever tech firm special-interests can get away with is going to happen. And as modern politics should make clear: there's not a lot of reasonableness left with which to restrain actors like that once things turn political.

Sorry, I didn't make it clear: I was arguing that point alone, not claiming they sold private data or anything else.

Ironically that Gruber comment was about the "user-enhancing" Sponsored Tiles episode from early 2014. That we are still seeing questionable and un-telegraphed behavior in 2019 suggests none of the privacy-relevant lessons were taken to heart.

Excellent way of shifting the goalpost away from your stance on the Booking.com case. :)

Displaying the same ad to everyone doesn't have privacy consequences. It's why billboards aren't privacy issues, nor are the TV ads, nor ads in a magazine. Ad networks are a privacy issue, self-hosted ads are not.