This could be caused by Google not recognizing and blocking Thunderbird's default user agent; try toggling general.useragent.compatMode.firefox to true (which basically has TB emulate Firefox's user agent)
> That's why we keep your verified mailbox address for sending mail; but there's no good reason to consider them different for the purpose of identity.
Both of Microsoft's own identity services (AAD and Live ID, and by extension O365 and Outlook.com) recognize (and allow creation of) [email protected], [email protected], and [email protected] as distinct identities/email addresses.
What you're saying is that once the first of (microcolonel|micro.colonel|mic.rocolonel)@example.com registers at github, the other two will no longer be able to do so, but will instead receive a confusing 'you already have an account' error, (hopefully) without being able to receive password reset emails.