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I believe the reason that Firefox disregards the target="_blank" attribute although it is totally legitimate to for us to use is that it has been configured to do this by the user. This can be found under Options / Tabs in the "Force links that open in new windows to open in" area.
The commonly Firefox Extension TabMixPlus, as well, has a special section that enforces that by default. See TabMixPlus Options > Links. There if the "Open links with a target attribute in current tab" is checked then the browser is instructed to disregard the target attributes and open those links in the same tab.
For many people this might be the issue, TabMixPlus. That option, to my mind, should default to not being checked as it is obscure to the user and the developer. However the reason for this might be an alleged phishing weakness from [secunia.com...] for Firefox 2.01...
Thank the folks that abused these controls and features in the past (think hidden off-screen windows, endlessly-spawning pop-ups, and redirect-or-load-on mouseover), but most or all of them can be disabled now. Some are configurable in the Tools-Options dialog boxes, while the rest are configurable at the about:config pseudo-URL, generally with option names including "dom.disable", "dom.allow", "dom.popup", or just "dom."
Jim