Perhaps you need to consider that using a word processor that is mean to place images on paper is not a good choice for writing an email where the content is sent as HTML 9Same as a web page). Thunderbird has an email composer, if you use it you will not pave these sorts of issues.

I am not aware of any fundamental changes on how Thunderbird pastes from the system clipboard. Perhaps Microsoft changed what they put into the clipboard with Word for Thunderbird to get. Are you using one of the online versions of word, they are limited over their desktop cousins although Microsoft don't like to admit it, they are not really the same thing.