Out of the box Firefox, even as-installed on Debian through apt, has sponsored content on the home page and new tabs. It's basically advertising.

There are other annoyances, I forget, every time I setup a new user or new profile with Firefox I'm reminded how much worse things are and have to go change a bunch of obnoxious stuff.

I miss Iceweasel.

Edit:

https://wiki.debian.org/Firefox#Disabling_automatic_connecti...