Besides clearing cache, clear the the temporary folders on your system. Check your Windows Task Manager for applications that you no longer want, and get rid of them.

Update your plugins - sounds like you may have done this, since you updated Flash.

  about:plugins  
  https://www.mozilla.org/en-US/plugincheck/

Update your extensions. about:addons Update all your filter subscriptions in Adblock Plus with Right click on ABP button > Filter Preferences > Action > update filter Reboot your system and bring up Firefox

I suspect it was the filter subscriptions and/or the reboot that fixed things on my system.

I updated anything else that needed to be, and did that update filter at Adblock Plus.

It's... a bit weird. When i try at youtube, it doesn't lag. When i switch to HD video it lags a tiny bit, and less when i let the bar disappear, but if i right click to display options it gets really really laggy.

On other sites it lags and skips frames even though the video is low quality.

Did you try to disable hardware acceleration in the Flash player?

You can check for problems caused by recent Flash updates and try these:

• disable a possible RealPlayer Browser Record Plugin extension for Firefox and update the RealPlayer if installed
• disable protected mode in Flash 11.3 and later
• disable hardware acceleration in the Flash plugin

• http://kb.mozillazine.org/Flash#Troubleshooting

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I have disabled hardware acceleration, protected mode, and realplayer plugin. Doesn't help. Default theme at appearance is what i already use. In Safe Mode there is also still no change.