At Tools|Internet Options, there should be a tab called "Programs."

There's an entry there for "HTML Editor." The program you put in there may affect appearance during your prep of a post, and not affect what comes back at you, through your browser, from the web? (Note that I don't know this to be true but it's a SWAG at a possibility.)

A usual program for that box is Microsoft Word - of course that assumes you have Word on your machine. Word should be able to use any fonts on your machine, but some other program might not be finding all of them?

A quick check on this post shows my "composing" box using Times New Roman, which is my default "Normal" font in Word, and it doesn't change if I go to IE Tools|Internet Options, and reset the IE font to something different - but the rest of the page does change. This implies that the post, when it comes back as part of the thread, uses the font I set in IE, but in the "typing box" the font selection comes from somewhere else, and I'll assume for now it's my Word setting that controls the input box. (????)

John