My username, along with the username I use across Mozilla, is Tannn3r. However, when I logged into my Marketplace account, I saw that my username was tannn3r2 and that tannn3r was taken. Upon further investigating, I realized that my account was possibly just duplicated, however I could not login to the tannn3r account. If you go to https://marketplace.mozilla.org/en-US/users/tannn3r2 it shows I have been a user since April 28, 2012 (which is incorrect). But https://marketplace.mozilla.org/en-US/users/tannn3r shows the accurate registration date, July 28, 2010. I am not sure if this is related to changing my email address last year or if it is something with BrowserID, but this is the only place I have noticed this.
It ended up just being a duplicate account on addons.mozilla.org, so I just had to delete the newer one.