[ubuntu] Suddenly, browser keeps crashing/freezing up whenever I use Youtube

I've been using the same version of Ubuntu on this same PC for a couple years now, and never had this problem before.
But lately, whenever I use youtube, even if I only have just one tab open, and one video playing, no matter how short the video is, it crashes my browser.

Specifically, it freezes it up. Strangely, the video continues to play while the issue happens, and I can still move the mouse around, however the problem is
that absolutely nothing I click on, in the browser, has any effect. None of the links or controls on the browser page or youtube page will work, when I click on them.
And I caot even quit or close the browser at all. So basically the video is stuck in the play mode, and I caot get out of it, or click to advance to another page.
In some cases the video does actually stop playing in the middle of it, in addition to all of the above issues, but I still am unable to move off of, or out of the page
by clicking anything at all.
The only way that I can escape out of this situation, is to Log Out of Ubuntu entirely, then wait a minute entirely, then log back in, and then re-open the browser.
Is there something defective about a flash player plug-in I have ? That's the only thing that makes sense to me, but I'd like to hear anyone else's suggestions.