Fortunately I don't have that version yet, but check under Tools/Options and see if there is an option to check or uncheck to alleviate the problem.
The Betanews site says, in part:
Firefox 52 also tightens security in a number of different ways. First, it displays a warning within both username and password fields on insecure (non-HTTPS) web pages. Non-secure websites can no longer set cookies with a "secure" attribute, and the Battery Status API has been removed to reduce the ability of trackers to fingerprint users.
So perhaps those insecure sites are HTTP, rather than HTTPS (many, many sites are that way). I don't know about your device (you didn't mention what it is).
That experience is one reason I never let software update automatically -- I keep control and know what's going on. You might see if you can roll back to the previous version. Most any FF version, 0.10.1 through 52, can be downloaded from Mozilla's ftp server at:
https://ftp.mozilla.org/pub/firefox/releases/