I've never heard of News Serif. Many of your correspondents won't have it either. So please don't imagine that your correspondents see your messages as you do. They will see it in either their default serif font (often Times New Roman) or they'll override it by one they choose to work with. Liberation Sans in my case.

I'm sorry, but I don't know the rules by which Thunderbird accepts or refuses to offer installed fonts, but generally when the question comes up, it means someone is using an unusual or arcane font and seems to think it will magically appear on everyone else's computer.

Can you point me to someone, or a document, that can show me how to get the News Serif typeface back?

It's on all the computers I use at home and at work, whether they're running XP, Vista, 7, or 8 as their OS and on all the MS and Adobe aps. It's still on my computer except in Thunderbird this morning when it disappeared. It was on all the emails I've sent for years.

Didn't you read the Message from Xenos, probably the only one seeing that font in your emails is you. Fonts do not get transmitted with emails so if a font is specified in email it will not be used unless the recipient also has it installed. Or do you only email yourself?

This is not a Thunderbird thing, it is an email thing, or more precisely a HTML thing.

Windows has at least two internal font lists, one display and one printer. It would not surprise me at all if there was a third... Web or HTML safe.

That does not include fonts windows gets as names from the printer and has no graphical display font for.

No-one else has stepped forward on this question about how to make fonts available in Thunderbird. Thank you Matt for your supporting comments.

Since you're talking about the font not appearing as an option (in a drop down?) I have to ask if you set this font each time you compose a message? If not, how did you know it had gone missing?

I would suggest:

a) go to the Config Editor and set (by hand) your chosen font as appropriate. You'll need to check which encodings you use and so which entries need to be adjusted. It's tedious to set them all by hand, and somewhat easier to use a competent editor on the prefs.js file and do a global find-and-replace. or b) use the Stationery add-on where you can create a template that specifically names the required font.

In browsing today, I discovered that what I thought was a fault in Thunderbird is actually an intentional design feature.

Even when you have chosen a default font, Thunderbird will by default omit it from its HTML headers, the intention being that the recipient's default font will be used. This alarmed me when I discovered that messages I intended to be sent in a sans-serif font were in fact being displayed to the recipient using a serif font, that being the default selection in Outlook where 99% of my messages are seen. Using Stationery means that I can ensure an explicit sans-serif type of font is named and used.

Your careful and deliberate selection of News Serif may have been futile. It would be instructive to get a copy of one of your sent mails, inspect its source and see if News Serif was explicitly summoned.