About a year ago I installed the chromium package provided by Debian and went to install my favorite set of extensions and kept getting a weird error about not being allowed to install them. Turns out that the Debian package maintainer had decided to build Chromium with a patch that disabled installing extensions unless you pass in a flag on the command line to chromium to re-enable the feature. I found this out not because it was properly documented but instead from a bug report on the Debian bug tracker.
Of course, FireFox on Debian hasn't been similarly modified to disallow installing extensions from AMO.
