mailto links should be handled by your OS's default mail client. I just realized mine opens Chrome even though I clicked it from Firefox and Thunderbird should be my mail client anyway.

yes, they should be. and they are (Mail opens) - but my point is the link opens in a new window which is blank, then brings up the Mail

I added an issue for you: https://github.com/mxstbr/sharingbuttons.io/issues/32