The problem is that I use keyboard shortcuts a lot. I find it easier to memorize a shortcut than to keep using the mouse.
In Opera, if you want to open a new tab (within a browser window), you press Ctrl+N.
In Firefox, if you want to open a new tab, you press Ctrl+T.
Ctrl+N in Firefox opens a new window, which is the convention that IE also follows. Ctrl+N also brings up a new document (in a new window if one is already open) in Microsoft Word.
When I want to open a tabbed window in Mozilla, most of the time I press Ctrl+N (because I’m a long-time Opera user) and then close the new window and press Ctrl+T. Sometimes, I catch myself but mostly I follow this roundabout method.
I don’t know who’s going to change their convention or if someone will. That’s not the issue. The issue is that inconsistencies across applications on common tasks inconvenience the user. There are some applications that won’t follow the Ctrl+C, Ctrl+X, Ctrl+V convention for copy, cut, and paste. They confuse the hell out of you, because a command that you’re used to will no longer do what you think it’s going to do.
Prior experience and perception, they’re big parts of the user experience. It’s something worth remembering.