This has been happening now for a few months, multiple versions of Firebug & FireFox.
After Firebug stops on a breakpoint, other Firefox tabs become unresponsive. A refresh of the page just gives the busy icon. Opening the frozen url in a new tab responds.
If I close FireFox there is still a process running. I have to kill it from Task Manager.
What seems to be the most reliable way to reproduce it is set a break point. When it activates, go to another firefox tab. It usually says the debugger is active in another tab - do you want to go there? When I'm done with the breakpoint I go back to that tab and it's frozen. A refresh just hangs.
One thing I have seen is that the mouse scroll wheel works to scroll, but clicking the scrollbar gets no response. No elements respond - menus, buttons, input fields etc. And the site being viewed doesn't matter once it freezes and any site being viewed can freeze.