MV2 extensions should work in Thunderbird without modification. I've made a couple of simple extensions that work like a charm in both Firefox and Thunderbird, but I'm not sure if more complex extensions would have issues. The documentation says thus:

Some information listed on MDN may not apply to Thunderbird and some API methods may not be supported. Each API page should include a compatibility chart and if that includes support for Firefox, it should work in Thunderbird as well.

Some MV3 extensions will not work in Thunderbird. See this page for list of missing functionality.