In the past month about 3 of my credit card sites stopped working on FFF, as well as my ISP's site. Some would flat out reject the agent ("Your browser is no longer supported"), others would let me log in but then immediately tell me I had been logged out or redirect back to the home page. So now I'm forced to open them back up in regular Firefox, history and tracking included.
It's one thing to say "Don't use sites that exploit your data", but it's not like the average person really has a choice when it comes to paying utility bills.