If I understand your question, you're using the Fonts drop-down and only seeing a few fonts.
I just tried this myself, and this is what I believe is happening. I have loads of fonts on my system, but I only see about half of them in this drop-down. My guess would be that the forum has a certain number of fonts that it can use, and the browser pulls up ones also found on your system. I chose Comic Sans and then increased the size a bit so you can see the result. I wanted to use one of my fave art deco fonts, but it isn't in the list.
If you have a Mac, I can't speak to that.
At first I thought it was only using True Type fonts, but many of my TT fonts do not appear in the list either. Well, I'll defer to Grant, who would know best.