Upgrade notification to Firefox 4.0.3 in the Unsafe Browser

On a fresh Tails 1.2.2 booted from ISO, after starting the Unsafe Browser, I get a notification suggesting me to upgrade to Firefox 4.0.3.

Feature Branch: bugfix/8694-disable-unsafe-browser-update-check

Original created by @intrigeri on 8694 (Redmine)

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