cpu 100% busy on youtube video starting from a list (but not on single videos)

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CPU hits 100% on ALL CORES with youtube videos starting from a youtube list, with only "Google adsense" reported on the page. Win7/64 + Firefox 3.6 (old version, needed for compatibility...). No issues spotted with adsense or any other trackers on any other site.
"Pause blocking" avoids the bug, every time
"Resume blocking" triggers the bug, every time (with every new video starting from list)
Starting the same videos without being part of a list also avoids the issue.
Symptoms are unusual in that OS functions seem affected
[1] all 4 cpu cores start busy-waiting, for a total 100% cpu busy when this occurs (4x ~25%, with some other random processes besides firefox reported spinning in Task Manager), and
[2] other running processes blip into foreground (and back), in front of firefox
[3] firefox itself behaves as if F11 was pressed on and off a few times
The machine becomes very unresponsive with all cores busy, with the "magic" unexpected windows gui actions very noticeable