You shouldn't allow Firefox to recommend things to you any more
The sad Firefox news of the time interval is Mozilla: Ad on Firefox’s new tab page was just another experiment, and also on Reddit. The important quote from the article is:
Some Firefox users yesterday started seeing an ad in the desktop version of the browser. It offers users a $20 Amazon gift card in return for booking your next hotel stay via Booking.com. We reached out to Mozilla, which confirmed the ad was a Firefox experiment and that no user data was being shared with its partners.
Mozilla of course claims that this was not an "ad"; to quote their spokesperson from the article:
“This snippet was an experiment to provide more value to Firefox users through offers provided by a partner,” a Mozilla spokesperson told VentureBeat. “It was not a paid placement or advertisement. [...]
This is horseshit, as the article notes. Regardless of whether Mozilla was getting paid for it, it was totally an ad, and that means that it is on the slippery slope towards all of the things that come with ads in general, including and especially ad-driven surveillance and data gathering. Mozilla even admitted that there was some degree of data gathering involved:
“About 25 percent of the U.S. audience who were using the latest edition of Firefox within the past five days were eligible to see it.”
In order to know who is in 'the US audience', Mozilla is collecting data on you and using it for ad targeting.
So, sadly, we've reached the point where you should go into your Firefox Preferences and disable every single thing that Mozilla would like to 'recommend' to you on your home page (or elsewhere). At the moment that is in the Home tab of Preferences, and is only 'Recommended by Pocket' and 'Snippets'; however, you should probably check back in every new version of Firefox to see if Mozilla has added anything new. This goes along with turning off Mozilla's ability to run Firefox studies and collect data from you and probably not running Firefox Nightly.
This may or may not prevent Mozilla from gathering data on you, but at least you've made your views clear to Mozilla and they can't honestly claim that they're acting innocently (as with SHIELD studies). They'll do so anyway, because that's how Mozilla is now, but we do what we can do. In fact, this specific issue is a manifestation of what I wrote in the aftermath of last year's explosion, where Mozilla promised to stop abusing the SHIELD system but that was mostly empty because they had other mechanisms available that would abuse people's trust in them. They have now demonstrated this by their use of the 'Snippets' system to push ads on people, and they're probably going to use every other technical mechanism that they have sooner or later.
The obvious end point is that Mozilla will resort to pushing this sort of thing as part of Firefox version updates, which means that you will have to inspect every new version carefully (at least all of the preferences) and perhaps stop upgrading or switch to custom builds of Firefox that have things stripped out, perhaps GNU IceCat.
(Possibly Debian will strip these things out of their version of Firefox should this come to pass. I wouldn't count on Ubuntu to do so. People on Windows or OS X are unfortunately on their own.)
PS: Chrome and Chromium are still probably worse from a privacy perspective, and they are certainly worse for addons safety, which you should definitely be worried about if you use addons at all.