I am experiencing extreme initial (first 60 seconds) hard disk activity while loading/watching HD clips with Flash in the latest versions of Firefox (problem started sometime after the fifth version and was never seen before). The SSD activity is so extreme that the computer can become unresponsive the first minute of load, after which videos play fine and do not lag. If monitored, one can see that Windows does not run out of memory (3 Gb; ≈1½ Gb free even during most severe loads).
Any HD video (even 1080p) loads and plays fine in both Chrome and IE. Disabling (starting in safe mode) plugins and addons tested: does not help.
x64 Ultimate SP1