Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b8pre) Gecko/20101203 Firefox/4.0b8pre ID:20101203030309 Installing an extension from a website which does not supply the content-length for an XPI, the download-progress indicator doesn't turn into undetermined mode on OS X. Steps: 1. Copy the URL 2. Open the Add-ons Manager 3. Paste the URL into the location bar and hit return The progress indicator will stay in determined mode.
Verified fixed with Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b8pre) Gecko/20101207 Firefox/4.0b8pre The litmus test work is already part of bug 553479.