I've seen the answer to this on a couple of other Q&A posts, but just in case...
On the Edit Profile page, under Username, it lets you delete the existing name and type in your own. And a helpful instruction tells you permissible characters. But if you attempt save the change, you get an error message.
The other Q&A posts say that you can't, in fact, change your Username. If so, why doesn't the Edit Profile page say so? Why does it mislead you into attempting to change your Username?
Also, the note under the Username textbox says, "Keep in mind that your username is visible to the public." So, if your e-mail includes some form of your personal name, and that name becomes your username by default (which happened to me), and you never notice this, you get exposed (such as by posting a query here)? But in fact, the "Keep in mind..." instruction isn't fully true. As one of the other Q&A posts says, if you choose a display name (which is blank until you add one), your username is NOT exposed. But the Edit Profile page doesn't tell you this.
So, the whole Username portion of the Edit Profile page is very misleading. It needs to be clarified. And, when you sign up for Firefox, you should be given an opportunity to choose a unique username. Mine was forced on me in the background, apparently.