Unfortunately gmail is just email login firefox account has nothing to do with what happens to your gmail. The primary login email if that gmail was it has to be cleared of inbox to receive the password reset otherwise there no other way to login. But if that is the case how are you posting on firefox if your gmail email login was used to login to your firefox account forum? When you reinstalled your O/S sync will not work if that computer was the main data stored for bookmark or data. So if your logged into your firefox account you can change the login but if your email is full you won't be able to update it. This is what I know of.

You can try a secondary email address in Firefox Account though not sure if you indeed still need access to primary email for verification.

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

I have a secondary email linked but it required main mail verification to login anyway and doesn't work with the secondary one. This account that Im writing this post from is another firefox account I created with another mail. Clearing inbox from emails on the first main mail doesn't help because the google disc is filled with my phone photos stored when I changed my mobile device. Is there any way to restore access to my account through my secondary mail perhaps?