The only way from inside Gmail I know is to right-click and it gives you the option to open it in an IE tab (assuming you’ve got the IE tab add-on). I just open it in the same page and use the back button coz I hate IE.
But, I’ve just discovered that if you have a Google personalised homepage, and have it display Gmail on there, you can make it open it in a new FF tab. I haven’t really tried it yet though, only tested it just now because I thought of it after I started typing this reply.