One other user reported this problem, but after a quick look it could be proved to be a firefox issue.
At first check if firefox is configured to launch external links into new tabs.
search for open_external
browser.link.open_external should be set to 1
if it's set to 1, then go to start menu and type:http://www.google.com
Firefox should launch Google in the existing tab. If it opens a new tab then there is some problem with Firefox on your system (maybe an addon?).
A clean install of Windows 7 and Firefox 3.6.3 or 3.6.4 does not open a new tab with browser.link.open_external set to 1.