That depends on many things. Are you using the current or older versions of Firefox? Is the data on all the systems, or some here-some there?

If you try to use sync, and the same entries have different values, you might lose the data you want.

Overall, you can change your "primary" email address for your Firefox Account quite easily thru Change... and Make Primary via Manage Account. See this KB article for more info: https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts

You may not be aware that with Sync / Firefox Accounts (FxA) you can add a secondary email address for transitioning your Firefox Account to a new email address, but that has to be done while you still have access to the currently known (to FxA) email address. https://support.mozilla.org/en-US/kb/add-secondary-email-address-firefox-accounts

Problem with that is that with an old email address you may need to be able to access that email account if you haven't used Sync for awhile or if you aren't currently logged into Sync on that device using that email address.