Flash under Linux?

Seeing as how people are experience troubles with Flash under Windows, I am almost afraid to ask... I am running FirefoxPortable 21 under Linux, and getting an error message saying my Flash plugin is out of date and is being disabled. Sure enough, no Flash is working on websites that use it. My question is how do I install the Flash plugin for Firefox under Linux. I tried downloading the Flash executable for Windows and running it under Wine, but it hung on me and was unable to complete.