Virus prevents antivirus updating
Alternatively, you can edit the file with a text editor.If you tried to change the channel by editing and there is no app.update.channel pref in about:config, or it has not changed, then the file needs fixing.
Some users may see a "Software Update Failed" dialog after restarting Firefox with the message, The update could not be installed.
This article describes how to configure and run Software Update and discusses problems that can occur during or after an update.
When an update is found, follow the on-screen instructions to download and install the update.
The application may be non-functional and/or result in your profile being corrupted.
See Profile_backup for information to safeguard your data, and see the daily builds threads at the Mozillazine forums for the current status of the builds: Firefox, Thunderbird.
The installer/zip/tar/dmg that you last used to install Firefox, Thunderbird, or Sea Monkey determines what update channel you start on.