As I usually say keeping things as short and simple as possible is the best way to go. On the other hand one's short way is other's long road so when it comes to closing Firefox tabs simply left clicking that "x" button present on every tab may not be the fast way since some people use the right click menu really often... so why not add a tab-related options to this menu? While Firefox doesn't offer such a feature CloseTab does exactly that... so here it is!
Since we're talking about a Firefox addon that's not touching one of those really popular parts of the Web browsing experience like social networking or YouTube videos downloads I am not surprised to see that CloseTab only has 3801 downloads so far. To make things the right way I will also add that current version labeled 1.2.1 was released on the 28th of March works with Firefox 3.6 - 4.0.* and finally it has 2 user ratings both 5/5 stars. Oh yeah if you're really that curious you should also know that CloseTab has a size of 5.1 KB and comes covered by the terms of the Mozilla Public License version 1.1.
Other than the above... I don't think there's anything special to say about the Close This Tab Open New Tab and Un-Close Tab options that appear in your Firefox's right click menu after installing CloseTab right? Just... I think that "un-close tab" sounds really strange maybe "reopen closed tab" or just "reopen tab" would be better if you ask me. That's all folks - I hope you'll find CloseTab to be more useful for your needs than I do - it's always left click on the "x" buttons or simply Ctrl+W for me! ;)