This is an intentional feature (enabled on Linux only) that is controlled by the hidden "middlemouse.contentLoadURL" pref. You can disable it by setting that pref to false using about:config.
Thanks, Gavin - The "middlemouse.contentLoadURL" does exactly what I wanted. Although I still think this "feature" should be disabled by default, particularly since there is no clear way to predict what will happen if the user middle-clicks on a link, it depends on how the link is implemented. I also think the name of the setting is a bit of a misnomer - maybe it should be "middlemouse.sendClipboardToSearchEngine"
(In reply to comment #2) > Although I still think this "feature" should be disabled by default, > particularly since there is no clear way to predict what will happen > if the user middle-clicks on a link, it depends on how the link is > implemented. This sounds like a bug that should be filed separately. > I also think the name of the setting is a bit of a misnomer - > maybe it should be "middlemouse.sendClipboardToSearchEngine" Well, we only send it to the search engine if it isn't a valid URL...
(In reply to comment #3) > This sounds like a bug that should be filed separately. I believe this bug report already addresses those concerns. > Well, we only send it to the search engine if it isn't a valid URL... Which is probably true of most people's clipboard 99% of the time. Even if the selection does contain a valid URL, it's still up to the user to try and guess what will happen when middle-clicking on a link.