Middle click for some reason works differently on *nix than is does on Windows. If you have a url in your clipboard, pressing middle click will load that page. In Firefox I normally use middle click to navigate (left and right for a large image or something).
Anyway to disable this do the following:
A Middle-click on a tab closes it on Windows by default. However, in Linux it pastes whatever is currently in the Clipboard and visits that site (or performs a search). This can be changed by setting the middlemouse.contentLoadURL pref to either true (for the Content Load URL feature) or false (for closing the tab).
Oh and FYI middle click for both new windows and pasting works simultaneously, here [Ubuntu]. It depends on context -- if it's a link, it'll open in a new window; if it's a textarea, it'll paste last-selected text.
Yeah but I normally hold down the middle button so I can scroll down (using an old school 3 button SGI mouse).
Solution: Use Ctrl + Click to open new tabs! Becomes habit soon enough, and works on 2-button mouses, too!
Be warned, however, that the definition of "clipboard" here is a little sketchy -- this isn't your usual Ctrl + C/X clipboard, but one formed from the previously selected text object.