Why a link is not opened in a new tab after left-clicking on a link while holding the Ctrl key?

I've got this problem since I installed the newest version (version 6.0) of Firefox. When I left-click on a link, holding the Ctrl key, then a link is not opened in a new tab, as it should, but in the same tab. It worked correctly in the previous versions of Firefox. However, I used the previous versions of Firefox on Windows XP. The problem occurrs on the newest version of Firefox, on a new laptop with Windows 7. The middle mouse-click works, i.e. still opens a link in a new tab. However, I'm using a Microsoft Natural Wireless mouse, where it's uncomfortable to use the middle click. I don't want to use another mouse, because this one is very ergonomic. I also use the Microsoft Natural Ergonomic keyboard.