Menu Editor 1.2.6 FTW! Rated 1 out of 5 stars

Add-Ons > Extensions > Menu Editor
On right list > File > Close Tab
Drag "Close Tab" from right list to Left List.

https://addons.mozilla.org/en-US/firefox/addon/710

Rated 5 out of 5 stars

probably my most used add-on. It should be in FF by default it's so useful. the FF3 version posted here by John Craick works great - thanks!

Works for FF3 (with some changes) Rated 5 out of 5 stars

I can also confirm that this works for FF3 with the changes yungster suggests. Thanks yungster!

version for FFX 3 Rated 4 out of 5 stars

CTC is perhaps my favorite extension. I can confirm that the recipe by yungster for making it FFX 3 compatible works ok.

I don't know how all this official extenion library stuff works but my modified version is available at ...
http://cid-c76e71a14d1e1a24.skydrive.live.com/self.aspx/Public/ctc-jc.xpi

making it work for FF3.0 Rated 5 out of 5 stars

You can make it work for FF3.0 by getting the source from the home page and changing some values. (I'd fix it but I don't know how to submit to cvs for this project)
install.rdf - change maxVersion to 3.*
ctc.xul - fix the menuitem label from "&closeTab.label;" to "Close Tab"

rezip the .xpi (see package.txt) and install it

Rated 5 out of 5 stars

this REEAAAAALLLLY has to be made FF3 compatible.

Thanks Rated 5 out of 5 stars

Exactly what I was searching for.