Rated 4 out of 5 stars

It's a great extension to replace Tab Progress Bar https://addons.mozilla.org/en-US/firefox/addon/tab-progress-bar/ not working on FF4 and upper versions in case you don't want to install the quite heavy Tab Mix Plus https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/?src=search which also support that progress bar on tab feature.
What is a bad idea is to use Progress Bar on Tab https://addons.mozilla.org/en-US/firefox/addon/progress-bar-on-tab/ because though it is a slimmed down version of this extension, it's causing severe conflicts with various important extensions (what suggest the slimming work is not well performed), you can see reviews there.

Also this extension induce misbehavior interacting with a couple of userstyles (for Stylish), Combine Tab Icon and Tab Close Button http://userstyles.org/styles/43992/fx11-combine-tab-icon-and-tab-close-button and with Collapse favicon, progress indicator & closebutton http://userstyles.org/styles/66179/collapse-favicon-progress-indicator-closebutton

This review is for a previous version of the add-on (0.4.2012020901.1-signed).