The Shockwave Flashplayer plug-in update is an executable file. When you download the update, you need to tell Firefox to '''save''' the file rather than '''open''' it. Then, after the update finishes downloading, you can close Firefox, use Windows Explorer to navigate to wherever you saved the update file, and double-click it to run the executable. If you don't know where Firefox is saving downloaded files, you can click through Tools> Options>Downloads and on the File/Windows tab, there is an option that lets you tell Firefox to always ask you where to save downloads. Enable that, and when the download starts, Firefox will ask you where to save it (by popping up the Windows file picker).