How to Disable Firefox Hello Completely
To disable Firefox Hello completely, you’ll need to change a hidden setting on the advanced about:config page. To access it, type “about:config” into Firefox’s address bar (without the quotes), press Enter, and agree to the “I’ll be careful, I promise!” prompt. Be careful with the settings you change here.
Type “loop.enabled” into the filter box at the top of the list of settings. You’ll see a “loop.enabled” preference which is set to “true” by default. Double-click it and it will be set to “false”.
Close and reopen Firefox and Firefox Hello will be completely disabled. To reenable it in the future, visit about:config and double-click the “loop.enabled” preference to set it back to “true”.
(“Loop” is the codename for the FIrefox Hello project, which is why all the Firefox Hello-related settings in about:config refer to “loop.”)