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This article describes how to upgrade Firefox version 1.0 or 1.5.
No matter which version you are upgrading from, the easiest way to upgrade to Firefox 3 is to simply install it from the mozilla.com website. Your bookmarks and other settings will be carried over automatically.
For details, see How to download and install Firefox on Windows.
For details, see How to download and install Firefox on Mac.
For details, see Install Firefox on Linux.