Billy,
there is an option for autoupdate.
app.update.enable
Type about:config in URLbar, agree with prompt and when it opens find this pref by search (type 'app.update' into search field)
look for the line
app.update.enable __ default __ boolean __ true
and double-click it to toggle, setting it to false
If you want it as default setting for all new profiles - find file \browse
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General