I can't recall a situation where an automatic update to a server,
OS, or piece of software improved its functionality or
usability. Best case - the update doesn't noticeably change
anything, and you carry on hoping the next update doesn't fuck you
over too badly.

On the other hand, I have plenty of anecdotes where an automatic
update resulted in worsened performance or usability, or even
breakage. On Linux servers in particular, I make sure package
updates are _never_ automatic.

Firefox seems to have taken the automatic update thing a bit too
far, in the latest builds it is no longer possible to turn off
updates in the user settings. They've completely removed all the
update options that used to be there, and now only allow you to view
update history or check for new updates. However, there is a way to
disable Firefox's updates by creating a policy file. Here is what to
do for a Linux or BSD host:

1) Create a directory 'distribution' under the top-level Firefox
install directory. In my case this was /usr/local/bin/firefox/, so I
just did a 'mkdir /usr/local/bin/firefox/distribution'.

2) Create a file 'policies.json' in that directory, with the
following content:

   {
     "policies": {
         "DisableAppUpdate": true,
         "DisableFirefoxStudies": true,
         "DisableSystemAddonUpdate": true,
         "DisableTelemetry": true
      }
   }

This is what appears to be the bare minimum to disable auto-updates
and turn off all the telemetry Mozilla turns on by default, however
you can change quite a few other settings (although note that the
telemetry can be disabled in the user settings).

I recommend the 'enterprise policy generator' addon [0], which
requires no permissions and will simply present a page of settings
you can toggle individually, while showing you the generated
policies.json that you can just copy/paste into the above
directory. It also displays help text on where to place the policy
file on Linux, Windows and Mac.

Once done, restart Firefox and in your update settings you will now
see 'Updates disabled by your system administrator'. You can also
see your currently active policy in 'about:policies'.

[0]: https://addons.mozilla.org/en-US/firefox/addon/enterprise-policy-generator/