The html css coding included in the 'HTML signature' is causing the problem. Usually, this can occur if you created the html signature using something like MSWord which includes a whole load of css stuff that bloats the signature file. It does not normally occur if the html signature file was created using Thunderbird. Any email that contains html css code influences the entire email and that can even effect replies. That's why it not a good idea to include specific font or font-size in an email because the recipient may not have that font on their computer or cannot read the email because they need a larger font. This same issue can also occur if someone received an email and tried to reply, but the sender included a load of html code that forced the code to be wrapped within a container and that container did not have a margin that was inset it from the actual edges of the email window. The image you supplied does not show the signature, nor the source code of the signature. But it has probably got a container which does not have a margin. I would use Thunderbird 'Write' window to create signature and then save it as a html file in a suitable location on the computer. Then select the option in Account Settings to attach the html signature file and choose the one you created using Thunderbird.