I believe this answers your question.

https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts

Well, hold on, if you change your email address, everyone who currently sends you emails will need to change to the new address. How quickly can you get them to switch?

In preparation, you can start by getting a new address, for example, from Apple, Google, Microsoft, or Yahoo, and then start telling people about it. Don't delete the old address just yet, but you can leave it for the junk mail subscriptions, etc.

What is more urgent is, if your email address was discovered along with any of your passwords, then you need to update your logins on any site using those passwords to keep your accounts safe.