Compose Responsive Images is a simple but effective Thunderbird add-on that makes newly inserted images in the compose window responsive by setting style="max-width:100%" (without overwriting existing styles if any) and removing width/height attributes of the <img> tag.
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From the Thunderbird Menu Bar, select Tools then Add-ons to open the Add-ons Manager.
Choose the Extensions tab, search for “responsive” and
look for Compose Responsive Images in the list (probably at the top).
Click the big green + Add to Thunderbird button then follow the prompts to install.
Author: Lionel Holt
Learned a lot from: Geoff Lankow