This is a rather simple problem, but it annoys me since some time. I'm using Firefox 35.0.01 on Ubuntu 14.10 and sometimes, when I open a new tab mainly on youtube, Firefox starts loading the page, but halts (the tiny progress bar is red and finished to three quaters) or slows down to an unusable amount. Then I have to minimize and maximize the window the make the page usable again or to view it at all. I expect it to be a problem with my hardware setup (I have 2G RAM) or with the current Adobe flash version. Maybe someone knows this behaviour or has similar problems.
It is from the flash I got 24Gb of RAM and got stuck with the same problem. I will try to find any solutions and come back with an answer for you ... any way for youtube is quite simple just switch to html5 player:
Just visit this page https://www.youtube.com/html5 after you login to youtube and change your default player to html5 player.