Open the Progress Panel (Window>Show View>Other>Progress) and click any red boxes you see there. What has happened is that FB is still compiling the project, but has already launched the browser window. Since the Browser really couldn't load the swf, which was still compiling, you're just kind of "stuck." Once everything cancels out, close the browser and click the "Run" or "Debug" button again. You should find that this time the file launches right off, as it was compiled last time.
I have the same issue on FB 4.6 when connecting to IE 9.0.8... for a debugging session. Can't get past 57%
Funny thing is - the environment WORKS FINE for many sessions - and then just stops working after a while.
Cleaning the project, restarting FB, killing tasks... all do nothing.
I can't right-click on the application to get the player's context menu (as suggested above), because the app is not yet running! IE just dings the system bell when I try that.
The workaround I've discovered - which works specifically for the situation described above - is to...
Keep an eye on the FB launch progress indicator as you start your debug session, after the blank IE window appears.
If you see it hanging at 57%, rapidly click the cancel and reload icons on the IE browser. The flash movie then loads as expected - still in debug mode.
I had the same problem with a web application in Firefox, FB 4.7, Win 7 64-bit. What worked for me was both of the following:
I've actually found a couple of scenarios recently where the debugger failed to connect that in retrospect should have been obvious, but weren't up front. These are worth checking for as well: