Outlook has an option to mark my comments with a whatever I specify. Does Thunderbird have an add-in to do this?
Use the Permanent pen feature of the MailTweak add-on to insert your inline reply with a different text style and colour. MailTweak will only change the text formatting and not throw in your name to preface your comments. But that should suffice to set your reply apart from the original text of the email.
If you wanted to use another add-on to insert your name, you can try and pair it up with Quicktext.
Lifehacker has a tutorial on the use of Quicktext. It basically consists of setting up templates and clicking or using a keyboard shortcut to drop in some auto-text.
Or just go the simple route of using your initials in all caps to mark the beginning of your response.
I found that it is also possible to reply in color using only the Quicktext extension with any standard text (e.g. name) in HTML.
E.g. "some text" in the Quicktext template will introduce red color "some text".
Multiple such templates of different color "red", "blue" and "green" etc. can be used.
One can then go back to any other color from the TB formatting tool bar (assuming HTML).