FIRST: OS 10.12.5--Mac OS of course! SECOND: Had to make a new account DoctorX linked to "gmail.com." My Sync e-mail is linked to "hotmail.com." I could not access my ancient The_Real_Doctor_X account I have had since 2011. I could not create a new account with "hotmail.com" since the e-mail is "taken" nor could I reset the password. IF that will create a problem with having "hotmail.com" rather than the "gmail.com" linked to THIS account as the Sync account, let me know. _____ REAL PROBLEM: Due to Firefox 53.02 shutting down all useful addons, I have tried the Firefox Developer. Neat! All work! I wanted to import via Sync. Set up Sync successfully with Firefox Developer. Neat! When I tried to do it with Firefox 53.02 . . . FAIL! I get the error page consistently. I have tried--with searching--to disable anything blocking cookies. I tried to find out how to allow "local storage." Based on a few hours searching, I made the settings you see. FAIL! Now, by copying the Profile attached to FF 53.02 into the Profile for FirefoxDeveloper, FirefoxDeveloper had ALL of the settings of FF 53.02 initially--including blocking third-party cookies--and FirefoxDeveloper WILL sign on to Sync. _____ WHY AM I DOING THIS: as you know every addon disabled by signing problems causes terminal dyspareunia throughout Al Gore's Internet. So I would like to occasionally use FirefoxDeveloper until these addons get signed. I HOPE with Synch my FF 53.02+ and other updates will be "synched" with FirefoxDeveloper so when these addons get signed I can mainly use FF 53.02 Edited to redact e-mail addresses.