I am sure that the current trend of changes to #Firefox and the resulting rewrite of #Thunderbird will necessarily affect #SeaMonkey. Essentially, I think SM lacks independent development resources, so if the divergence is quick enough or large enough, SM will have to write a separate mail component or give up.

This reminds me of a Matrix client I was using. The developers of what was then called Riot changed frameworks or something and the downstream client I used ended development.