All Firefox releases (i.e. not nightly builds) can be obtained from the Firefox release repository.
The various options for telling Firefox what to do with respect to updates are available from about:preferences. Exactly where in the options these settings are available depends on the version of Firefox you’re using. In some versions of Firefox you are not permitted to make changes to these settings, or which changes you can make are restricted. If so, you may need to make changes to values in about:config.
If you can’t make the changes from about:preferences, in about:config, you can check/change any of the following, depending on what you desire: app.update.auto, app.update.enabled, app.update.service.enabled, and app.update.silent. If you can make the change from about:preferences, it’s recommended that you make any change there. If you can’t, then iteratively change the above about:config settings and see what the new settings show in about:preferences (so you know what the equivalent is of the new values). You can restore defaults to values in about:config by using the “reset” selection from the options context menu (right-click).