Mine was already set to "false" There are a ton of "update" categories. I set a bunch that were true to false (not any that applied to "extensions", just "app").
Yes. I very well may have set the appropriate flag back in version 32, and don't remember which it is. I wondered about the "false" value I had. But you're doing what I would if it were a problem for me.
I recently discovered a reliable method that can be easily implemented (and also easily reverted).
Quit Firefox and delete the contents from the ~/Library/Caches/Mozilla directory. Then click once on the Mozilla directory and run the shortcut cmd i for the information in the Finder. Click the "Locked" option there. Done.