The answer to "what is Firefox?" on Mozilla's FAQ page about its browser used to read:

The Firefox Browser is the only major browser backed by a not-for-profit that doesn’t sell your personal data to advertisers while helping you protect your personal information.

Now it just says:

The Firefox Browser, the only major browser backed by a not-for-profit, helps you protect your personal information.

In other words, Mozilla is no longer willing to commit to not selling your personal data to advertisers.

A related change was also highlighted by mozilla.org commenter jkaelin, who linked direct to the source code for that FAQ page. To answer the question, "is Firefox free?" Moz used to say:

Yep! The Firefox Browser is free. Super free, actually. No hidden costs or anything. You don’t pay anything to use it, and we don’t sell your personal data.

Now it simply reads:

Yep! The Firefox Browser is free. Super free, actually. No hidden costs or anything. You don’t pay anything to use it.

Again, a pledge to not sell people's data has disappeared. Varma insisted this is the result of the fluid definition of “sell” in the context of data sharing and privacy.