There are two different versions of Flash, the ActiveX version for IE and the Plugin version used by most other browsers.

1.Download the Flash Plugin version setup file from here:
Adobe Flash - Plugin version.
Save it to your Desktop.

2. Close Firefox using File > Exit
then check the Task Manager > Processes tab to make sure firefox.exe is closed,
{XP: Ctrl+Alt+Del, Vista: Shift+Ctrl+ESC = Processes tab}

3. Then run the Flash setup file from your Desktop.

4. Start Firefox and test your installation here: https://www.adobe.com/software/flash/about/

• On Vista and Windows 7 you may need to run the plugin installer as Administrator by starting the installer via the right-click context menu if you do not get an UAC prompt to ask for permission to continue (i.e nothing seems to happen).
  

See this:
http://vistasupport.mvps.org/run_as_administrator.htm

I did this before 100 times, but nothing has changed. I did not have this problem until the other days.

Make sure that you have a 32 bit Flash version for a 32 bit Firefox version and not a 64 bit Flash version.

See:

• http://kb2.adobe.com/cps/000/6b3af6c9.html

I done this several times and nothing is happening. Strange thing is that until other days was working just fine.