I'm using the latest version of Firefox for macOS. Tab 1 is open to, for example, the Webcam Girls forum. I open Tab 2 to one of the threads in that forum. When I use Cmd-1 to go back to Tab 1, if Tab 2 has not finished loading, Tab 1 acts like I clicked on the Home link at the top of the page. Clicking on the tab with the mouse works as expected, and it only seems to happen if it's Tab 1. It's rather annoying behavior, but I guess all I need to do is make sure I use Tab 1 for another website.