Been having problems for ages where if I have firefox open all day, it starts to slow down until eventually it grinds to a halt and the whole computer needs to be restarted. (Thankfully it wasn't affecting any other apps, so I'd just abandon firefox for the day). I was using firefox rather than Safari as Safari has a similar problem, except it slows down the whole computer!

Have disabled the flash plugin and now it flies all day, woohoo! Just a bit of advice if anybody else is having a similar problem.

Didn't you try click to flash? That stops flash unless you click on the link. It makes what ever you run as stable as possible. Though I find having loads of tabs and window open has the same effect with Safari if left for a long time.