This extension has become extremely deprecated. It often will get an IP/DNS entry stuck in it somehow, and keeps reporting the same location from an earlier site for all subsequent sites until/if it gets unstuck. I am going to uninstall and just use FlagFox, FlagFox just works.
After install on FF 36.0 Whois Lookup & ... appeared correctly in the add-ons list, but does nothing. At least nothing you would exspect like adding a button to your add-on-bar or an option to the context menu. So, in my case it provided no function, nothing useful, just nothing. What it did, as I realized watching the HTTP response headers, was excessive spying. Each and every page I visited.was instantly phoned home (home to Key Systems, Germany, who are hiding behind the "myip.ms" profile. For whatever reason.) and sent to Google Analytics - together with a suspiciously long, encrypted parameter list. So to put this in clear words: This plugin does not just send queries about websites, you want examined, it sends everything you do to Key Systems AG as well as to Google Analytics. I, for one, would not exactly call this "moderate use".
I installed this add-on on Firefox 33.0.1, but it did not work, no flags or buttons appeared (it was listed in the add-on manager, though). Then I tried to uninstall it, now my address bar is gone and session restore and customise UI don't work anymore. It essentially broke my browser, I'll have to restore my profile somehow.
updated to 29.0.1, extension icon is covered by a message all the time (message about page loading status) maybe you fix it? or make it a regular button icon - i'll just place it into my status bar (it's an extension now, mozilla removed status bar :p) then