My favourite and most useful browser plug-in is by far PrefBar, which I have been using since the early days of Mozilla. Recently I needed to spoof the user agent string and found that there is no Flock plug-in to do it. Since a UA spoofer is part of the PrefBar I tried to install it and see what happens. Lo and behold it works.

To install the PrefBar in Flock, visit http://prefbar.mozdev.org/installation.html and copy the URL of the 'Install PrefBar' link, paste it in Flock's address bar, hit ENTER, and it will install like any other plug-in. Simple.