One of those add-ons that do exactly one thing and to it well. The only thing I don't like is the fact that the keyboard shortcut Control+Q is hardcoded and can't be changed.
It should also be noted that for Windows, AutoHotkey provides an easy way to get this functionality system-wide and put it on any keyboard shortcut you like, making this add-on unnecessary. But that's just for information, and doesn't reduce the usefulness of this addon for those who don't want to use AutoHotkey.
@drhouse: (Belated) thanks for the info about AHK and Greasemonkey in your reply. I use both, but of course they're not for everyone. Most people won't be willing to dabble with script stuff themselves. That's why I still think that it would be a good idea to make the shortcut configurable in your addon. :)
If possible it would be great if this worked in the regular firefox search box, rather than only on web pages. IMHO the text currently in the clipboard should be checked to see if it is already in quotes, and if so, they shouldn't be added. Also (and this could be quite important) nQuote should not replace the text currently on the clipboard if it is just being used to add quotes to already selected text.
As "No Nothing" says, the nQuote option should really be hidden when not in a text field or text area. I've started looking at the code myself, and it looks like the hidden state of context-paste isn't getting picked up, so probably best to just use pq.hidden = !(gContextMenu.onTextInput) in your code. However, a few other things (position in the context menu, key accelerator) aren't getting set in addition to this, so the script could do with some debugging.