An assumption: because you updated it to 23.0 the options may have been reset. Go back into your options (tools, options, advanced, update tab, and click on "Never check for updates", then click OK at the bottom. That should fix it. If not, please post a reply here.

If you are running more than one Logon User Account in Windows (each has its' own Firefox Profile) or have more than one Profile in Firefox in that user account, you need to make that setting in all user accounts or Profiles that use Firefox on that PC. "Never Check for Updates" is set per Profile

From the start I uninstalled ff 27 beta something. Got version 23.0. Installed it. Went to options and checked do not update. And it updated anyways to. 26.0 a day later.