You have a Mailfence account, and are using Thunderbird to manage it. Your storage allotment is managed by Mailfence.

Thunderbird is a email client that you can manage one or more email address in like @gmail.com or the thousands other email service options out there.

Thunderbird is not a email provider nor a server as there is no official @thunderbird.xyz email for public to sign up on and use or such.

Neither the company Mozilla or the Mozilla Foundation's subsidiary MZLA Technologies Corporation that makes the Thunderbird email client has any form of email services for the public. Mozilla does have *@mozilla.com/*@mozilla.org email however they are only manually provided to those who work for Mozilla in a capacity that warrants getting one as they have to give it up when they leave and are not available to the general public.

Btw this is a Support Forum and nobody here does replies by publicly posted email address and do post a subject for your support forum threads instead of just .....