I may be off topic but I'm on subtopic.
I've had that happen to me a few times too. It always came as a complete surprise and I could never figure out afterwards what I'd done or how to undo it.

However...

I looked it up. In Firefox 2 at least, if I'm trying to type an uppercase W but I accidentally hold down Ctrl instead of Shift, the window (or tab) that I'm working in will close, poof!

There's also a remedy. I can click on History in the menu bar, then on Recently Closed Tabs in the pulldown menu, and get a list of tabs I've closed recently with the most recent at the top. Clicking on it reopens it. Firefox (this one, anyway) has been pretty good about restoring not only the tab but most of my post as well. If I get a blank text box instead, usually just waiting a few seconds will make the post reappear. I think I may occasionally have sped up the process by right-clicking (in the text box) and choosing "Undo" but that might just have been how long it took for the post to restore itself.

As I understand it, Ctrl-Shift-Tab has the effect of restoring the most recently closed tab.