User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0) Gecko/20100101 Firefox/4.0 Build Identifier: Thunderbird 3.19 When a file is selected for a signature to be added to email, at the top of the image is -- and a space. The image or company logo follow under this. it is then necessary to delete this added text at the insertion point. Reproducible: Always Steps to Reproduce: 1.Default signature file selected under TOOLS/ACCOUNT SETTINGS 2.New mail is started 3.Logo appears with the added -- at the space above

Yes, that's on purpose and this separator marks the start of a signature. Thus, e-mail clients can display the signature with a different color or remove it again, e.g., when replying. The upcoming TB 3.3 version (currently in alpha stage) will have a hidden preference setting to disable the signature separator for an account.