Also, it happens even if you don't do anything with browser.

Try to set the Flash plug to click-to-play by setting the permissions for Flash to "Ask To Activate" in "Firefox/Tools > Add-ons > Plugins". That way you can activate the Flash plugin when you need it via the Lego block icon on the location bar.

You can check for problems with current Shockwave Flash plugin versions and try this:

• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)
• disable hardware acceleration in the Flash plugin

See also:

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

But it is not a solution for this problem because mozilla never had a problem with Flash plugins. It appears in the latest versions.

I opened everything what I had here in Chrome and There is no problem with freezes or lags or flash.

Google Chrome uses is own Pepper based version of Flash, so you can't compare this behavior with Firefox.

serega3306, do you intend to be testing the first Beta build of 31.0 and not using the Firefox 30.0 release?

I already use build 31.0 and because of it I said that Firefox stops to collect to much memory and shut down after 2 GB memory and begin to freeze every 3-5 minutes when I watch a video or open 2 tabs of youtube video(one stays in background download and second video plays in normal mode without acceleration). Even now I got freeze when scrolled this page. Yesterday, I opened onlineTV and firefox freezed every 1-2 minutes, but memory collecting was 700 MB whole time.