How are you describing this GIF? and where is it located? See https://stackoverflow.com/questions/56827235/what-does-moz-do-not-send-do

Unless you consider yourself very good with HTML, I suggest you compose your signature in Thunderbird as an email, save it as a HTML file and use the file in Thunderbird for your signature. Suport document https://support.mozilla.org/en-US/kb/signatures#w_signatures-stored-in-files

The code for the signature is on the HTML side of Signature Switch. The code I am using is right out of their own example:

img src="{{Digital Twin}}" border="0" width="400" height="100"

I removed the <> from the above so that this post doesn't use the HTML.

I did a quick test, inserting this into the HTML sig box;

img src="file://C:/Users/dskir/Downloads/2024-12-15-15-48-38-073890.gif" width="400" height="100" alt="" title="" /

It does not appear properly in compose window, but appears properly in receipt of mail . I suggest testing this with some friends, as users may find it distracting. Some of the visual experience may be reduced for users who use dark themes with email.

Thanks