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.
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.
Fixed in bug 616641 - https://hg.mozilla.org/mozilla-central/rev/63fc6076169b
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.