Firefox only shows about 80 characters for the tooltip preview. For example, the first sticky post “Have a Computer question?..” (yeah, I know there’s some irony in using that example) shows a tooltip of “Over the past few months we have received a spate of computer related questions in GQ”
In IE I see two whole paragraphs.
I couldn’t seem to find where that setting is, any help would be appreciated!
You need the Long Titles extension.
Thanks Q.E.D. - that worked. 
If this is a very well known issue for Firefox (which it seems to be,) why hasn’t it been fixed yet? Is it merely that most webpages are written incorrectly, so they work with IE, but not with other browsers (which I hear is a common problem)?
Probably because it’s not a critical problem, so it’s low on the priority list.
In fact, it is (supposedly – I haven’t installed it myself) fixed in FireFox 3.0, which is now in Alpha. From the Long Titles Extension page posted by QED: