http://fpdownload.macromedia.com/get...player-win.xpi
open that file with winzip or whatever you use for archives.. if you don't have anything just rename it to flashplayer-win.zip and windows should be able to handle it
extract the files out and copy or cut flashplayer.xpt and npsw32.dll go to start.. run.. type in "%appdata%" without the quotes.. then find the mozilla folder and the plugins folder inside of that and paste those files there
restart firefox
this will also work in the firefox portable version (run from a usb drive or similar) but is a different folder obviously (FirefoxPortableAppfirefoxplugins)