This has been going on for a while now. When there are new updates for Firefox add-ons and I start Firefox it's trying to download them but it fails. E.g. the first extension is Firebug. After a while I get a dialog:
Firefox could not install the file at
http://releases.mozilla.org/pub/mozilla.org/addons/1843/firebug-1.6.0-fx.xpi
because: Download error
-228
How can I fix this?
If the extension is installed via the package manager, you won't be able to download updates. Uninstall it using the package manager, and install from the repository.
The repository doesn't update versions between releases, but security patches are backported. If you have multiple users, you may want to use the repository as all users get the extension. Otherwise, each user (and profile) needs to download and install their own copy.
Some bums didn't update the add-on URL correctly.
Get them here manually: ftp://ftp.mozilla.org/pub/mozilla.org/addons/
Disable ipv6.
That should fix your problem.
BTW, to avoid confusion, keep in mind your updating an extension, not a plugin. Plugins in Ubuntu are usually installed and updated via repository. Some extensions can also be installed/updated via repository, but most commonly they are downloaded and updated via Mozilla Add-ons site and Firefox Add-ons Manager, which is what you are trying to do.