Firefox now enables its "sync" feature by default. Apparently the developers think Google is an innovator and they want Firefox to be just like Chrome.
Formerly a user could copy a Firefox folder with bookmarks, cookies, add-ons and customization from, say, a Linux PC to the "users/joeblow/appdata/roaming" folder on, say, Windows 7.
Now Firefox doesn't recognize that folder and prompts the hapless user to sign into a "Firefox account," the sole purpose of which is to "sync data among devices." You can turn sync off in "about:config" but the folder still isn't recognized. Also, when you try to sign-in to the "account" (e.g., to make sure no data got into the cloud by mistake) the login just hangs, because the Firefox team is trying to suck data off your PC with no sync capabilty.
Also the only way to delete your browsing data is by deleting your entire "Firefox account."
But - hey, it's OK -- all that personal data in the cloud is going to be super-duper encrypted.