Double check to make sure that you have taskbar grouping
set to "Always combine..".
You said that it worked like this though:
Quote: Originally Posted by ourfear
also, I put the whole firefox directory to driver C: , do same action as above, it works as it should.
It could be a bug since this is still a pre beta version of Windows 7. You should send a feedback report
to Microsoft to help make them aware of it. Hopefully they will fix it in a newer version.