That could indicate a missing font-based icon, the letter being the corresponding character in a different font that Firefox is using instead. Have you noticed any issues with downloadable fonts, icons, or emojis on Yahoo or other sites?

My problem is difficult to describe. When I receive an email, the email address is PRECEDED by a very fancy CAPITAL letter which is usually the Initials of the person writing. This began several weeks ago. I'm not sure if YAHOO started it or Mozilla---please try to find out. In ONE case, however, the writer's initials are TR and the capital letter is an H which isn't at all related to that person. Where did the H come from????????? AND: HOW CAN I CHANGE IT TO THE CORRECT LETTER?

My Yahoo inbox doesn't look different, but when I open the message, there is a little image to the left of the sender name. Is that new? It seems to be a feature of the site.

I mostly use this mailbox for subscriptions, so many of them are logos; one is a product image. On a spam message from someone named "William Jonh" whose email address started with w, there is a "W". No surprise there. Not enough data to explain the odd one you see.

Thanks for the reply, Josh, but you have not helped me CORRECT the error. The H appears before a contact with the Initials T or P Regan. The H makes no sense and I want to get rid of it!!

Thank you

I think the letter is chosen by Yahoo. If you want to make extraordinary efforts to modify a particular icon on a page, you could consider a user script. (That's a script you can inject into the page using an add-on like Tampermonkey, Violentmonkey, or Greasemonkey, to override its normal behavior.) Alternately you could use a custom style rule to hide ALL of the icons. (For example, using the Stylus extension.)

I will add STYLUS to Mozilla and try to eliminate all the unnecessary identification letters.. Thanks for the suggestion. Meanwhile try to find out how Yahoo initiated this nonsense without my permission!!!