Works really well, should have compatibility Rated 4 out of 5 stars

This addon works very well for displaying quick information about your ISP's net usage!
If you are able to understand how it parses the webpages, you can even add a new ISP to the list.
The one drawback is that the author doesn't submit the newest versions to AMO, forcing people to go to an external website and get a version which has not been reviewed. This also means the version on AMO is often incompatible with the latest version of firefox (as is the case now).

This review is for a previous version of the add-on (1.2.337.1.1-signed).