Go to www.portableapps.com and download and install Firefox portable to a USB memory stick.
Look for two files with names like these these in the plugins directory of your installed version (usually located at C:\Program Files\Mozilla Firefox\data\plugins) :
Navigate to the portable version on your memory key at:
/portableapps/firefoxportable/Data/pluginsAnd copy those files into that directory.
Restart the browser and it should work.
If you do not have access to an installed copy of Firefox you can download an older version of Flash as a zip file and extract those two files. However, you will likely get out of date warnings when you want to run it.