So you have tried upgrading both Firefox (from 12 to 17) and your RealNetworks add-ons already?

Here's my list of "other things to try" which may or may not be covered by the sources you used:

(1) Disable protected mode (Win Vista & Win 7)

See this support article from Adobe under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?

(2) Disable hardware graphics acceleration in Firefox and in Flash

(A) In Firefox, un-check the box here and restart:

orange Firefox button or classic Tools menu > Options > Advanced > General > "Use hardware acceleration when available"

(B) In Flash, see this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues

(3) For video issues, make sure your RealNetworks add-ons are the latest versions

(4) For display artifacts while scrolling, disable Smooth Scrolling here:

orange Firefox button or classic Tools menu > Options > Advanced > General > "Use smooth scrolling"

By the way, are you in the HTML5 trial? https://www.youtube.com/html5

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

"Clear the Cache":

• Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Press and hold Shift and left-click the Reload button.
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (MAC)