Learn how to open new tabs when typing URLs into the address bar in many browsers.

I don't know about IE9 but Firefox 4 has "Open new windows in a new tab instead" checked by default. I assume IE does the same things which explains the behavior you're seeing.

You can easily open a link in a new tab on your Mac computer by right clicking or using a keyboard shortcut.

Firefox add-on adds an option to open a link in a new tab without loading the page until you switch to the tab.