I had auto update turned on, so I thought it would auto update automatically.
If update is found,then Firefox starts downloading in the background without disturbing browsing ie when Firefox is not loading any tabs.
It will take sometime to finish downloading as it only uses bandwidth ,when Firefox is idle(not loading tabs) and download stops, when you exit Firefox.
In your case, Firefox was downloading updates and when you checked in "about Firefox", Firefox just resumed the update from where it stopped.
Note: Firefox uses Mozilla maintenance service to do this.
Manually Updating Firefox Configuring Automatic Updates Reinstalling Firefox Community Q&A Firefox is normally configured to update automatically in the background, so that you don’t need to worry about keeping up to date.
If these settings have changed, however, you may be wondering how to update the program.
I am little confused as to how does Firefox auto update works?
So, lately, Firefox had a bug in 38.x version - so they recommended to update to the latest version.
And people with high speed internet will not even know that update has taken place.
Mozilla Firefox is one of the web browsers that is continuously updated by its developers.
As soon as a new update is released and becomes available for the end users, the users are instantaneously notified about the released update, and are prompted and strongly recommended to download them in order to stay protected from the external attacks and to improve the efficiency of the web browser.