Some people blame the new plugin isolation code that just went into Firefox 3.6.4, you could try temporarily turning off plugins entirely by starting firefox this way:
in your desktop's menu editor so when you start Firefox it starts in safe mode.
If the system suddenly gets better you'll have evidence that it's the flash plugin + the IPC feature that's at fault. You could then
just turn off the IPC feature in the about
:config page in Firefox: