A program can not be running to do an update.

If you can, do any updates when the browser is not in use.

Well then shouldn't the browser know this – by looking at processes running on the machine – and tell me? "You have Firefox running in another user account. Close it before running this update."

It shouldn't just cause an error.

Oh and the error shown is incorrect. I am not using an older version of Firefox. There's only one version on the machine, in /Applications.

You are using a beta version of Firefox and it updates often. You might want to use the normal version that updates only now and then.

Download Firefox Full Version For All Languages And Systems http://www.mozilla.org/en-US/firefox/all/ https://www.mozilla.org/en-US/firefox/releasenotes/

I've turned off auto-updating for now. I'll let Firefox tell me when an update is available and do it manually, ensuring that only one copy is launched.

I still consider this a bug which should be addressed.

 if (other.firefox.process.exists.on.this.machine) {
   don't update;
   warn user;
 } else {
   update;
 }

You can file a bug report here: https://bugzilla.mozilla.org/

See Bugzilla etiquette before commenting in bug reports. https://bugzilla.mozilla.org/page.cgi?id=etiquette.html

Roger that, will do. Thanks.