I see you're using FIrefox ESR. If that Firefox doesn't have this functionality don't think there is another one that implements this.

Setting each Firefox to not update itself for all users (manually) it's not possible for what you need, no ?

Firefox -> Options -> Advanced -> Update : Check

       Never check for updates. 

instead of

         Automatically install updates.

You need this to work on how many machines ?

Please note that running Firefox without updates is a pretty critical security risk for your company. I would research to try to find a better solution than that if I were you.

It's not that we are never going to update Firefox. We are going to update when we can get a service window. Alle of the clients are not local admins, and therefore they can't update. Try to imagine 100 users, calling the IT dep. saying that they can't update firefox as it is asking them to do.

I found an article describing an update service that can be disabled, during install. The article also states that this service is implemented in Firefox 16. Anyone that knows, when this version is being released?

Link to article : https://wiki.mozilla.org/Installer:Command_Line_Arguments

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Currently Firefox 16 is in Beta Channel and will be releases official on October 9, 2012. https://wiki.mozilla.org/Features/Release_Tracking#Firefox_16:_Desktop