Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove the Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > Cookies: "Show Cookies"

Forgot to mention that I have tried that a few different times and did not solve the problem.

I've also tried uninstalling then reinstalling the flash plugin like some have suggested when google searching, but that also did nothing to solve it.

Thanks for the reply however!

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

In Firefox 4 you can use one of these to start in Safe mode:

• Help > Restart with Add-ons Disabled
• Hold down the Shift key while double clicking the Firefox desktop shortcut (Windows)

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

You can also try "Reset all user preferences to Firefox defaults" on the Safe mode start window.

I tried all the suggested items listed and none worked. Even in safe mode youtube videos still gave the same error message in Firefox 4.0.1.

I have yet been able to compare with another Mac with Firefox 4 installed, hopefully I will be able to do that soon to see if I have anything different in the settings and such. If not, it may for some reason be a hardware issue (?) or possibly some settings in Snow Leopard that are messing it up versus it being a problem with Firefox.

• Also installed and tried out Firefox 5 beta and same thing occurs with youtube videos.

Allow cookies observation: If you allow cookies from "youtube.com" instead of "http://www.youtube.com" videos will play without having "accept 3rd party cookies" checked

Removed and reinstalled Firefox... problem solved.... Also got rid of the "another version running" flash screen on startup and updated to v13...