Rated 3 out of 5 stars
It does not appear to work at all. No picture with a peer index number appears on twitter.com.This review is for a previous version of the add-on (0.9.8).
In the previous version you had to switch the plugin on. To do this click on the off word next to the PI logo near the search box on Firefox 4. The latest version (uploaded in last couple of days) defaults to on.