Are you using a 64-bit architecture? When I was running 64-bit Firefox and using nspluginwrapper to run Flash, I was getting total browser lockups and 50% (100% of one CPU) usage in some (totally random) instances of Flash usage. It got to the point where I downgraded to 32-bit Firefox with 32-bit Flash, where I haven't had as many problems (but still some laggy Firefox performance, for example on Buffalo Wild Wings' website). Actually, I would guess that because of issues like this and the Java plugin, Ubuntu probably uses 32-bit Firefox even for 64-bit architectures.
