Hello marchopkins,
1. Open up a new email in thunderbird
2. Add any text you would like in your signature
3. Copy and paste your companys logo wherever you would like it in your signature in the open email.
4. Click on File - Save As - File
5. Save this as an HTML file
6. Go to tools then Account settings
7. Click the Attach this Signature text box
8. Choose your signature HTML file
9. Click ok
You can test this by opening up a new email and your signature should be attached.
Let me know if you have any questions or are still having trouble.
Thanks