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If you follow the instructions at that should work for you.
I don't want to install other email reader (even if it is part of the same project) or browser, or are you saying that using those guidelines we can updated firefox and thunderbird with no installation of new programs?
Here are the instructions how to enable Java Script in your web browser.
When you first set up your email accounts, Thunderbird configures itself to get new email automatically.
So no, you can't just use that repository and update Thunderbird.
There is a way to update thunderbird manually, but you have to download the packages from Mozilla directly.
If you want to do the update manually, when you feel like it or when you know the all important plugins you use are compatible with the new version, you may follow the instructions to disable automatic update of Thunderbird and Firefox. I found that previous version of Thunderbird and Firefox are not always easy to find on the Mozilla Website.