If you’re trying to either install Adobe Flash into a Firefox install on a computer you don’t have admin rights on, or install it into Portable Firefox, then you’re probably having trouble. The solution is relatively simple considering Adobe Flash runs off of only just two files! Just copy those files and you’re set.
Download the flash installer, then use a program such as WinRAR or PeaZip to “extract the .EXE file”.
Then find the files flashplayer.xpt and NPSWF32.dll which you already extracted, and copy those to the folder Moziilla Firefox/plugins.
Restart Firefox, and it’s really as simple as that!
Another option if for some reason this doesn’t work though it should, is to download this extension and install it inside firefox: http://fpdownload.macromedia.com/get/flashplayer/xpi/current/flashplayer-win.xpi