Ditch Firefox... Switch to Swiftfox
Trying to install Flash was such a pain in Firefox. I couldn't find 'flashplugin-nonfree' in the repository and there were posts about some problems with 64bit. Then I came across this reccomendation to try Swiftfox. Its built on firefox and tons faster and meant for Linux..
Easy install... and boy, did it ever work! Went to a flash page and it asked to install the plugin.. and zap! flash was installed! google video here i come!!