Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b9pre) Gecko/20110104 Firefox/4.0b9pre ID:20110104030354 When I download add-ons on a slower machine, I can see a lazy initialization of the download progress bar. The progress starts but gets reset to zero within the next second. Further the width of the whole notification is changing a couple of times. See the screencast: http://screencast.com/t/vT5I2kkR Steps: 1. Install any add-on, i.e. Cooliris 2. Watch the doorhanger notification
Do you see this on all platforms? I don't see it on OSX and your screencast is Linux.
Looks like that on Linux the notification is not wide enough and the download progress info below the bar is the reason why the width gets updated.