It should be there if the font is installed on your computer. Thunderbird just provides you with a list of the fonts installed on your computer, so if this font disappeared at some point, it was likely something to do with Windows and not Thunderbird. I hesitate to suggest it, since many people will latch onto it as an excuse for the next 500 things that go wrong with their computer, but possibly it was removed with the Windows 8.1 update? Maybe there was some kind of licensing spat between Microsoft and whomever owns the rights to that font which required that it be removed. Not saying that is what happened, but it's a possibility.
I have the latest 24.2.0 version of Thunderbird (TB) email client (and Windows 8).
Until a few past versions of TB were introduced, Edwardian Script ITC used to be among the menu of available fonts. Then that option disappeared. Is there any way to add that font option to TB's font menu? (When that elegant font used to be available, I used to like using it for sending emails containing messages like "Happy New Year." )
Happy New Year to the the entire immensely knowledgeable and most helping crew of moderators at this forum!