Mozilla Firefox is an open-source web browser, so anyone can take its source code and modify it. Various projects have taken Firefox and released their own versions, either to optimize it, add new features, or align it with their philosophy.
These projects all have to release the source code to their browsers and can’t call them Firefox or use official Mozilla branding, such as the Firefox logo.
Mozilla doesn’t provide official builds of Firefox compiled for 64-bit systems yet. Waterfox takes Firefox’s code and compiles it for 64-bit Windows, without adding additional features or making other changes. Many plugins, including Adobe Flash, now have 64-bit versions, so using a 64-bit browser for day-to-day browsing is very possible. If you’ve already got Flash installed, you may need to download its installer to get the 64-bit version, too. The current installers come with both 32 and 64-bit plugins.
Waterfox uses the same profile data Firefox does, so switching to Waterfox is easy. If you decide to uninstall it, don’t select the “Remove my personal data” option unless you also want to delete your Firefox data.
Pale Moon is another “optimized” build of Firefox for Windows, but it also has a 32-bit version. Pale Moon diverges from Firefox in removing accessibility and parental control options, while modifying the default interface settings to be similar to earlier versions of Firefox — it has a bookmark toolbar and status bar by default. It also uses its own configuration directory, unlike Waterfox.
SeaMonkey isn’t technically based on Firefox, but it’s closely related. Firefox was the evolution of the “Mozilla Application Suite,” which also contained email, IRC chat, HTML-editing, and newsgroup capabilities. These features were ripped out of Firefox to make it a more focused, speedy Web browser. If you long for the days of Mozilla, you can use SeaMonkey, the successor to the full Mozilla suite. It’s also got an integrated feed reader.
If you’re using Debian Linux, you probably have Iceweasel installed instead of Firefox. Mozilla won’t allow Debian to package and tweak their own version of Firefox without calling it something different, so Iceweasel was born. Iceweasel is functionally identical to Firefox; it just has a different name and logo.
IceCat is the GNU version of Firefox for Linux and other free operating systems. Mozilla Firefox is free software, but it recommends non-free, closed-source software such as the Adobe Flash plugin. The Free Software Foundation didn’t like this, so they released their own version of Firefox, which doesn’t recommend installing non-free plugins. IceCat is identical to Firefox beyond not recommending proprietary software and changing the branding, although it also includes an extension that makes a few privacy tweaks.
Wyzo is optimized for downloads and online media. It includes multi-source download capabilities and an integrated BitTorrent client. Its start page contains links to easily search torrents videos, TV shows, and music. Unfortunately, it hasn’t been updated in a while and is still based on Firefox 3.6.4. You can get many of its features in Firefox by installing extensions, such as FireDownload and FireTorrent — but these extensions also don’t support newer versions of Firefox, either. Still, it’s an interesting concept.
You may also have heard of SwiftFox, an optimized build of Mozilla Firefox for Linux. It hasn’t been updated since the Firefox 3.6 series, so it won’t offer you improved speed. Linux distributions package their own builds of Firefox, which are optimized for 64-bit operating systems.