> It's just that when the address bar already has text, your click may be mistaken as a selection if the pointing device simultaneously moves slightly. It's easy to lose the selections. The ease of accidentally overriding the selection seems like a fundamental limitation of the select+middleclick technique.
This is why Linux native browsers, like Konqueror, include a "Clear address bar" button. You can get extensions for Firefox for this sort of thing, too. I find it useful enough to use this feature even when not middle clicking.
In FF it's not really required anyway: Middle click in any part of the page will attempt to navigate to the text you last selected, if it looks like a URL. Address bar is not needed.