Firefox shouldn't do this you may also want to run through your settings, (hit the left alt on your keyboard when in the browser) tools, etc.
You probably just had something enabled that should be. This is a dota2 forum not tech support though.
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Sometimes when I Ctrl+Click a link it will open it in the current tab AND in a new tab. Only seems to happen when I either search for a player and click on the results or I open a profile and click on the recent games. Not sure if it's just me or if there's some weird javascript at play here. Only happens when I browse in Firefox (version 21).