Works with FF6! Rated 5 out of 5 stars
Actually, I'm using it with Pale Moon 6. But I think it should work with FF6, using the same procedure.
Since v6 hides the Menu Bar by default, installing this extension might seem to be pointless, especially since it doesn't show up on the context menu when right-clicking on a tab. However, by installing Menu Editor -- https://addons.mozilla.org/en-US/firefox/addon/menu-editor/ -- and enabling the advanced mode, you can copy the menu item for URL Lister from the Tools menu to the Tab Context Menu. Sweet!
I'm glad that it worked, because Tab URL Copier and CopyAllURLs both say they're incompatible with FF6.