“Mozilla Executive Chair Mitchell Baker announced […] that Thunderbird development needs to be uncoupled from Firefox. [Due to] developers spending large efforts responding to changes to Mozilla technologies” - Wikipedia
Implementing WebExtensions (and deprecating everything else) would definitely be such a large effort. I doubt that’s going to happen in a software that hasn’t seen any major changes in years.
You can still use ctypes on Thunderbird and SeaMonkey add-ons. There are no plans to implement WebExtensions APIs on either of them.
Ah, I hoped to switch to WebExtensions and forget about the ctypes… 
Does that mean, that I still need to maintain both versions of the extension?
What is the best approach to switch my Firefox users to WebExtensions without losing the auditory of Thunderbird and Seamonkey?
There aren’t many good options at present and I suggest that you hold off taking any action for now. We’re talking to the Thunderbird Council about some possibilities and should have something we can announce in the next month or so.
Thanks Jorge. Having solution for Seamonkey would be also welcome.