flash player won't install for firefox only

chrome and ie works.  ff is the only problem.When i get to the window that says "do you want the  following program to make changes to your computer" i hit yes and nothing happens. I tried to download it again but it says  "only a single application of this program can run." either i have to go to files to delete install_flashplayer11x64_mssid _aih.exe or restart my computer to try to download flash again. I have to restart it because sometimes it says " adobe flash player installer is running" but i can't find it. I tried disabling firewall and turned off all my plugins and nothing works. also i am the administrator on the computer

adobe flash player 11

windows 7

64-bit operating system

don't know if browser is 32 or 64 bit and i have no idea how to check