User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

A tab crashed. I clicked the restore tab button.

Actual results:

It told me that I needed to restart the browser because Firefox had been updated in the background. Despite the fact that I've told it not to do that in my preferences.

Expected results:

The tab should have been restored, and Firefox should have continued without updating itself.

Possibly relevant - I was running multiple profile instances concurrently. Could it be that the update preference is per profile, but that the update itself affects all profiles? So one profile may update the browser in the background, even though the profile I'm currently using is configured to not do that?

If so, it's a bug to have the preference be per profile. At the very least, that setting should be inherited when a new profile is created so that the user doesn't see unexpected behaviour. Or better, that preference would be shared by all profiles, so it's not possible for them to have different values.

Yes, the auto update preference is per-profile on Linux (only on Windows is it per-installation). If you want to set it for the whole installation, create a distribution directory inside the Firefox installation directory, and create a distribution/policies.json file, with AppAutoUpdate set to false:

{
  "policies": {
    "AppAutoUpdate": false
  }
}

General info on the policy mechanism is in the policy-templates README.

Instances updating while the other is running is bug 1480452, work is underway to fix that and related issues in bug 1553982.

Implementing a per-installation setting on Linux like this would be fairly involved, we generally don't assume Firefox has write access to its own installation directory, and there isn't a handy system-wide location for a setting like this to be written that we could rely on across distros. As we don't have any plans for that I'm closing this as WONTFIX.

But there is a handy system-wide location for setting things like that, and has been for many years. It's defined in the XDG Base Directory Specification (https://specifications.freedesktop.org/basedir-spec/latest/ar01s02.html) and is supported by every major distribution. You can write settings like that under $XDG_CONFIG_HOME.

Now admittedly, that would be per-user, rather than per-installation. But that's vastly preferable to the current situation of per-profile.

I agree that per-user would be preferable to per-profile for this setting, and thanks for pointing to the spec. I'm going to reopen this to indicate patches would be considered, but we would need to carefully weigh the maintenance cost of a new profile-external configuration for this purpose.