Firefox 3 comes pre-loaded as part of Ubuntu Hardy Heron, but if you're using an older version of Ubuntu or a different Linux distribution you may need download and install Firefox yourself.
There are several ways to download and install Firefox to your Linux system.
The easiest and most versatile method if you're using Ubuntu, is to go to Applications > Add/Remove.
In the search bar, type Firefox, and press enter on your keyboard.Using the check boxes, select Firefox Web Browser, and any add-ons you would like.
When you're finished, click Apply Changes.
You can also download by going to the Firefox website. If your security settings don't prohibit it, the Firefox website will auto detect your operating system, and suggest the correct Firefox version.
Click the button that says Firefox 3 Free Download.
When the download window opens, select either Save File or Open with. If you choose Open with, be sure to select your preferred package manager from the drop down menu.

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OK. When the file is finished downloading, install in the manner you prefer.