I'm not a techie, but that sounds like a problem I had with an earlier version of Flash Player, which I cured by disabling hardware acceleration in Flash Player. To check this out,
1) call up a youtube video, and pause it.
2) right click the video. You should see a drop down menu.
3) scroll down to "settings" and you should see a small box appear in the screen. If there is a check next to "enable hardware acceleration," click on it to clear it.
4) close out the small screen, exit Firefox, call it up again, and try viewing a video to see if it lags or crashes.
Along this line, the newer versions of Firefox also use hardware acceleration. You can also try disabling that to see if it cures the problem. Hardware Acceleration in Firefox was causing display problems with my brand new HP Omni 22-1125, in spite of the fact that it has an up to date Intel graphics driver, etc. (When I'd right-click a menu, such as bookmarks, if the menu extended toward the bottom of the screen, overhanging the taskbar, it would leave an image on the taskbar when the drop-down menu was closed. I'd have to click on the "show home page" button to clear it.)
1) Click on the Tools menu.
2) Click on Options
3) Click on the Advanced tab.
4) If "hardware acceleration" is checked, click on it to remove the check.
5) Close out Firefox and restart it to test.
I guess it made a slight difference, but the problem is still there. YouTube seems to work somehow (although laggily and with large frame skips), but any other video site is almost 100% bound to cause a quick crash.