Hi,

Please see this.

I uninstalled Realplayer, and afterwards the soundcloud files played as before in firefox 13.0.1

Okay, updating the flash player plugin did not really fix it. It got a little better for flash players (obviously) but even the html5 player is lagging...

Try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://hacks.mozilla.org/2010/09/hardware-acceleration/

• https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers

Try to "disable the hardware acceleration" in the Flash Player.

• http://helpx.adobe.com/flash-player/kb/video-playback-issues.html

See also:

• https://support.mozilla.org/kb/keep-flash-up-to-date-and-troubleshoot-problems

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html

Nope, didn't help.

If you have security software you can try to re-trust Firefox + its sub processes fully in the different modules in both the antivirus and firewall software. Please see this.

AV/Firewall Configuration

You can also temporarily try to open Flash sites inside Firefox using IE Tab + or IE Tab whenever the problem occurs.

Windows Firewall has an exception for Firefox's plugin container but not for Firefox itself. Avast doesn't have an exception for anything but I doubt it's the problem as I had Antivir before when the problem was already there.

You can try to adjust the auto sandbox, browser sandboxing and safe zone modules. The Avast forum would also be helpful.