Willing to report a crash bug - don't know what's causing it

Hello,
recently it often occurs that Firefox crashes completely (i.e. unexpected close) and I have to restart Firefox.
On the one hand, it does not affect me that much, since most of my tabs are restored after the restart, but on the other hand I'm still convinced that it should not happen at all :)
Now regarding the bug: The crash happens when I open more than 2 tabs and directly close¹ one of the tabs, but it does not happen every time - this is just the most fitting way to describe it.
Maybe there's a way for me to find out more about this issue and finally write a useful bug report which can be easily dealt with.
I also hope that I do not report something that is already known - I've read through a lot of bug reports with some not-understandable titles, so that I didn't feel capable of figuring out whether this issue has already been reportet.

Kind regards,
Frederik "ChosenOne" Braun

P.S.:
I'm using Firefox 3.0, the one officially shipped with Ubuntu.
¹) I don't know if it's important, but I always use the middle mouse-button for closing.

EDIT:
I ran Firefox from a console (screen is so comfortable :)) and watched the output. The last message is "Segmentation fault"....great. How do we get more/verbose output? :)