If you design and build sites, then you will find this plugin extremely useful:
https://addons.mozilla.org/en-US/firefox/addon/60
* Web Development: Adds a menu and a toolbar with various web developer tools.
It is extremely useful while you are developing your sites to check for issues or compatibility etc...
Works in Flock 2.0, I am sure it works in flock 1.2.1 as well...