Firefox on Ubuntu – Slowing Down?

For the last few days I’ve been having issues with Firefox on Ubuntu.

Browser windows / tabs become unresponsive for no obvious reason until I end up having to restart it…
Flock, on the other hand, is quite fast and responsive.

Is this a common issue or do I have a buggy or memory hogging plugin installed?
 I think I’ll be removing all the plugins this evening to see what happens, but one thing is for sure Flock is becoming more and more attractive!