Rated 5 out of 5 stars

Could we please have this solution too to be done via update manager:
copy and paste this into your address bar and it will do the update for you ---> https://github.com/downloads/athomas/mozilla-firefox-addons/gmail_manager-0.6.4.1.3-fx+sm.xpi

Unless a person comes to this page, he/she can't really know what went wrong, except that the add on is broken.

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