"Back in the day" I did use the old manual Sync trigger button for manual operation of "Sync-at-will only", so I think that I know what you are looking for.

No hidden pref in about:config .

There is a Sync Now button in the drop-down menu from the Firefox Accounts button on the Navigation toolbar, but when the user disconnects from Sync (using Sign Out) that menu item disappears. So that "manual trigger" ain't gonna work for what you want to achieve.

Year ago that Sync Now button WAS the manual trigger for doing a manual Sync, in the days of early Australis versions of Firefox 29 up thru the Fx38 version. I see it in the ESR Fx 38 version I have installed and use on occasion when I want access to an older non-Quantum versions of Firefox.

the-edmeister said

"Back in the day" I did use the old manual Sync trigger button for manual operation of "Sync-at-will only", so I think that I know what you are looking for. No hidden pref in about:config . There is a Sync Now button in the drop-down menu from the Firefox Accounts button on the Navigation toolbar, but when the user disconnects from Sync (using Sign Out) that menu item disappears. So that "manual trigger" ain't gonna work for what you want to achieve. Year ago that Sync Now button WAS the manual trigger for doing a manual Sync, in the days of early Australis versions of Firefox 29 up thru the Fx38 version. I see it in the ESR Fx 38 version I have installed and use on occasion when I want access to an older non-Quantum versions of Firefox.

Thanks for confirming. I've made my peace with most of the lost extension functionality, but this one still drives me crazy. Ah well, will have to learn to live with it.

The Sync credentials are stored in the Password Manager and the only way not to get a master password prompt when you start Firefox is to disconnect Sync and only connect when you want to sync. In most cases canceling the MP should work although any action to access the Password Manager or visit to a website with a stored password will generate a new MP prompt.

cor-el said

The Sync credentials are stored in the Password Manager and the only way not to get a master password prompt when you start Firefox is to disconnect Sync and only connect when you want to sync. In most cases canceling the MP should work although any action to access the Password Manager or visit to a website with a stored password will generate a new MP prompt.

Thanks. Yes, this is precisely the issue, multiple MP prompts being generated when I don't want or need to input the password. They'll pop up even on sites where I am already logged in. So I either type in the password needlessly, or I keep canceling the prompt. I have been looking for an alternative to the Master Password+ add-on I used for years with Firefox and Thunderbird, but no luck. It's the one issue with post-XUL Firefox that still drives me nuts.

The only way to prevent unnecessary MP prompts is to disable the Password Manager temporarily (you need to reload the page to trigger the Password Manager).

• Options/Preferences -> Privacy & Security: Logins and Passwords: "Ask to save logins and passwords for websites"

You can consider to keep the Options/Preferences page open in a pinned tab for easy access.