Recently, I was setting up Firefox on a new PC and had trouble logging in. I couldn't remember my password and kept making new ones. This resulted in me losing all my settings and most importantly, shortcuts on a new tab. I had everything organized but because of the login troubles, now all of it's come to waste. Because of that, I'm suggesting primary syncing devices - you could set a device to be your primary device, and when syncing, all other devices use the primary device's settings. When on a secondary device, the settings you change will only stay on the secondary device, and get marked as "specifically for this device" or you can go to the syncing page and manually sync the device's settings with your primary device, making the changed settings transfer onto the primary device. Alternatively, you could make the secondary device the new primary device, overwriting all other devices' settings with the new primary device's settings. To prevent what happened to me, there could be a pop-up which appears when you turn on sync, asking you whether or not you want it to make it the primary device. You could also preserve primary status on devices which have been logged out of, waiting for you to either log in on the device or set a new primary one. Maybe you could even make syncing work cross-platform. Have settings on your pc and your phone be the same! But with the new specified settings feature, you can still have differing settings if you wish so.
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