Go to your user profile for that website and change the e-mail.

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

Hi, sorry for not being clear yes I'm talking about sync.

If this is the case, why can't I re-use the deleted email?

I don't know for sure, but don't worry. If you change the e-mail, only the data on the sync is removed. The data on your computers will not be changed.

Hi Fred,

I'm not so concerned about the data on my PC. Basically, I have 2 profiles set up in account:profiles and would like one synced with email A and one with email B.

Email B was initially the primary email for my work profile, but I changed that to Email A and removed email B. I want email B to be used as my personal profile, but it is seems as though Firefox sync still has it linked to my work account as a secondary email, despite having removed it.

Not sure how to handle that. It might be better to start over. Remember that the data in your computers is safe.