Is Mission Control the same thing as Spaces? I have the possibility of using 4 different desktops, but Firefox appears only on one of them. I am using Snow Leopard, not Lion. Have you checked the Application Assignments in the settings in System Preferences?

Thank you for the reply. Yes, Mission Control and Spaces appear to be very similar, but Mission Control is an "improved" feature in Lion. I agree with you, Firefox worked fine for me using Snow Leopard. But now I'm losing my mind trying to stop my Firefox windows from appearing everywhere. I know my colleague is not experiencing this, and he is running the same versions (Firefox 11.0, Mac OS X 10.7.3) as me. Is anyone else having this problem? Suggestions, please?

Well, I have to admit I am rather disappointed with the lack of response to this issue. I figured it out myself. For those of you with the same problem (I see five of you according to the counter here) I hope my answer helps. Mac Lion provides Mission Control and has deprecated Spaces. With Spaces, one could assign an application to different desktops. Now you can right-click on an application’s Dock icon and look under Options. Here you can assign to All Desktops, This Desktop, or the default- None. For some reason my Firefox was set to "All Desktops" (and drove me crazy for several months!).