Never post any personal information because an adware robot could find it, and start sending you all kinds of spam. This includes e-mail, phone, and home information.

Please remove that information from your post.

What Firefox account are you trying to change?

However, if you are talking about sync;

Resetting the sync password, or changing its e-mail, will remove all data from the servers.

This is a security feature. https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

• I've lost my Firefox Sync account information - What to do
• I’ve forgotten, or don't have access to, my Firefox Account email address

foldenra1 said

Some years ago I changed my primary gmail account from [email protected] to [email protected].

If you are referring to a Firefox Account for Sync, allowing a secondary email address is a recent change, like within the last 5 or 6 months. Previously the user needed to create a new Firefox Account when they needed to use a different email address. And "some years ago" with the 2 previous versions of Sync / Weave a new account was needed too, when a different email address was needed.