If you use Firefox 3 and Yahoo Mail (the nice new interface, not Classic), you might be dealing with a bug I've noticed where the message pane scrollbar doesn't display. It's not a game-breaker, as you can still use the page-down and page-up keys to navigate, but it's an annoyance.
It was a mystery bug for me, as only my primary workstation suffered from it. Two other systems had no issues. Disabling all addons on my afflicted system didn't change the situation. Neither did starting in Safe Mode, which surprised me.
I had been using a kludgey workaround with the useful IE Tab addon to render Yahoo mail with the IE engine (but still within the Firefox window). That worked fine, but it means that any links I clicked in Yahoo mail also loaded with the IE engine. Not terrible, but not optimal (though IE tab is still a great addon for IE-specific sites).
So I did some more digging, and found a suggested workaround from others who have dealt with the same bug: try a different browser theme. And lucky me, I found a few that fix the issue.
The best-looking theme (per my tastes, of course) I found that brought the scrollbar back is Sky. Aeon and Aeon Colors also work and look good. If neither suits you, head to Mozilla's list of Themes to try others - but you'll probably have to try a bunch to find those that bring back the scrollbar. I did.
Aside from changing themes, I also found a few mentions that said the Stylish add-on fixes the problem. But it looks like you'd have to configure it to change the style for the Yahoo Mail page, and I wanted something that worked right off the bat without further mucking about. It's Friday, after all.
It might also work to uninstall and re-install the browser, but again, I wanted an easy fix. And adding a new theme is plenty easy (though it would be nice if you didn't have to restart Firefox to apply it).