What about this? https://thunderbird-webextensions.readthedocs.io/en/68/compose.html

TyDraniu said

What about this? https://thunderbird-webextensions.readthedocs.io/en/68/compose.html

This doesn't add entries to message folder. It just opens a composing window.

Your question is a bit ambiguous.

Sounds like you want to 'move' a newly downloaded message from eg: 'Inbox' to a specific folder like creating a message filter. But you say "Is it not possible to create new messages? " hence the provided link to 'compose'.

Sorry for being ambiguous. I wanted to scrape a website for updates and show them in similar way RSS feed are shown in Thunderbird.

Bibek Panthi said

Sorry for being ambiguous. I wanted to scrape a website for updates and show them in similar way RSS feed are shown in Thunderbird.

then scrape your web site to an RSS feed on the web and display it as an RSS feed.

However this is not really the place to ask questions about how to program extension or use the Thunderbird's web extension sub/super set. While the documentation calls them web extensions, mail extensions is a more accurate name and they are different to the browser focused web extensions and they are evolving at a faily rapid pace.

I would suggest you try the four other locations that are given in the docs for help with webextensions. https://thunderbird-webextensions.readthedocs.io/en/68/index.html

Please see the "Where To Get Help" heading at the bottom of the page.