WHen the THunderbird 5 update is started, it asks "This package needs to download original setup files from LiberKey servers. Download size: 13.4 MB Do you wish to continue"

I press "Yes", the files download, installation starts.

After 15 minutes, that Thunderbird install window is still open and never closes. After an hour, it is still open.

I cancel, retry the whole operation, still doesn't finish.

Download the LKS package ("offline setup" which is not really offlline for Thunderbird/Firefox, doh!) and double-click on it and "infinite loops" would end