After digging around trying to find some way to increase performance of the Adobe Flash Player Plugin v10.0, I found a culprit.
This plugin provides a wrapper for legacy Netscape plugins and seems to, quite unnecessarily, stick a layer of overhead on top of the flash player plugin.
a) check you have a 32-bit machine, as a 64-bit machine is going to need the wrapper regardless.
b) check you have no use for nspluginwrapper (its package provides the command 'mozilla-plugin-config'. This will show you what plugins are actually wrapped)
$ mozilla-plugin-config --list
Original plugin: /usr/lib/mozilla/plugins/libflashplayer.so
Wrapper version string: X (1.3.0)
c) uninstall nspluginwrapper using your distro's package manager (typically apt-get or yum)
d) restart your browser
...after removing the offending package, I noticed that my CPU load, when playing a YouTube video, dropped from ~95% to ~60%. Not as good as the ~30% when using mplayerplugin, but workable nonetheless. Also, full screen video was watchable albeit with nearly full CPU load & a little shearing.