Upon opening my Firefox browser as usual, the window suggested creating a Firefox sync account, a concept similar to Google and Microsoft’s single sign-in accounts. Firefox sync allows users to log on to Firefox using the same account across multiple devices. It literally allows users to access the same sessions as and when they swap devices.
Firefox sync helps users access bookmarks, tabs, and passwords with one easy sign-in. The data is encrypted so no one else could access it without logging on to the account. This is important for people are usually more conscious about their web history than anything else.
All accounts that are logged on from one device are logged on simultaneously on other devices. Eg. If you logged on to your Facebook account on your laptop, it would be accessible from your phone on which you have signed on to Firefox.
How to set up & use Firefox Sync on your PC
1] Click on the menu button on the top-right corner of the Firefox browser and then on “Sign in to Sync.”
2] If you have already created an account from the main device, you could sign-in with your username and password, else you could sign-up for a new account. 
3] The bookmarks, tabs, passwords, etc are shared from the main device, the one which is synced with Firefox first. You can choose what exactly to sync.
4] To add more laptops and desktops to the list of devices, simply sign-in to Firefox on them and allow them some time to map the data.
Remove a computer from Firefox Sync
1] Click on the menu button on the top-right corner of the Firefox browser and the first option would be your Sync account name, usually your email address. This will lead you to the Sync preferences page.
2] Click on Disconnect. Your device would be removed from the list of synchronized devices.
3] In case you wish to Disconnect the device while using the primary system, please clickon Manage devices on the Sync preferences page. 
Note: A device appears on the list only when synced. The device read the instruction to unsync only when the system is connected to the internet.
How to set up Firefox Sync on your Android device
1] Tap on the menu button on your Firefox browser, which is usually 3 dots on the top-right corner. In some devices, the option could be below the screen.
2] Select Settings from the list of options and then click on Sign in.
3] You can either sign-in or sign-up with a new account. The rule remains the same that is the device is the first one you sign-in to the account with, is the one from which all the data is synced further.
Remove an Android device from Firefox Sync
1] Open your Android device’s Settings menu (not the Firefox browser’s settings).
2] Check for the option Accounts or Accounts and Sync and open it.
3] Among the list of apps, choose Firefox. It will show the list of accounts which have been logged on to Firefox for Android on your device.
4] Tap on the menu (the 3 dots on the top-right corner of the screen) and select remove account.
How to set up Firefox Sync on your iOS device
1] Tap on the menu button on your Firefox browser, which is usually 3 straight lines on the top-right corner.
2] Tap on Settings and then Sign in to Sync.
3] Either create a new account or sign-in to your existing Firefox account. If this is the primary device you are signing on to, the data would be synced from the device itself. Or else it would map the data from the primary device.
Change primary email address on Firefox Accounts
1] Click on the menu button and then click on your Firefox account to open the Firefox settings.
2] Click on Manage account.
3] Select Change next to the secondary email and then choose Make primary.
You can sign-up for Firefox sync her at mozilla.org.
See this post if Firefox Sync is not working.
