No, Sync accounts can't be merged per se. The Sync server isn't set up to work in that manner.

But if you disconnect from one Sync account and you are able to log in to the 2nd Sync account on that same device, your data will be merged in that Firefox installation. You may experience duplicated data, and I not sure how the Sync server will handle that situation.

Before you do that, backup your Profile data in case things go pear-shaped. https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles#w_backing-up-your-profile

If you can access this secondary Sync account then you can delete this account if you want to add this email address to your other account as a secondary account. If you do not know the password for this account then do a password reset.

• https://accounts.firefox.com/signin
• https://accounts.firefox.com/settings