I think it's app.update.service.enabled [false] and/or app.update.auto [false]
My Firefox portable which does not update, has both these toggled, then on the General settings page it reads "Firefox Updates", "Updates disabled by your system administrator"
If you want to disable updating add-ons and extensions too, then AFAIK it's also app.update.service.enabled -and- extensions.update.autoUpdateDefault that need toggled.
However it doesn't seem like it should be locking up for 5 minutes. Are you running it off a USB flash drive, other slow removable media, or a very slow embedded SSD? I'm not trying to convince you to continue updating it, though many who want to keep same version, will instead go with an ESR release version or the portable version. I'm not suggesting you "need" to do that... what I might do instead is just disable auto-update then manually update it when you can accept the 5 minutes downtime, are away from the system, unless you need to keep old add-ons working with an older version of FIrefox.