After months of delays and well over a year in development Mozilla has released Firefox 4.0 on March 22nd. Firefox 4.0 is the first release built off of the new Gecko 2.0 rendering engine and is much faster than previous versions. Firefox 4.0 features a redesigned user interface along with many more new features that are listed in the release notes.

Guides & Tours

I have put together a A Quick Guide to Firefox 4 which covers some of the unique changes in Firefox 4.0. Mozilla also has an interactive tour on their site.

System Requirements
Firefox 4.0 will work with:

• Windows: 2000, Server 2003, XP, Vista and 7
• Mac: OSX 10.5 or 10.6 and an Intel Chip
• Linux: requirements will vary by distro package

Installing Firefox
The download from the Mozilla site is an installer NOT an upgrade. Presently Mozilla does not offer a direct upgrade from Firefox 3.6 to 4.0. Running the installer using the default setup method will install Firefox 4.0 over Firefox 3.6. If you want to keep Firefox 3.6 installed see the post Installing Firefox 4.

Update: With the release of Firefox 3.6.16 on March 22nd, there is now a way to upgrade from Firefox 3.6 to Firefox 4.0, see the post Firefox 3.5.18/3.6.16 Released & Update to 4.0 for more information.

IMPORTANT: while it is possible to revert back to Firefox 3.6 (if still installed) doing so after using your profile on Firefox 4.0 may cause issues. See this post for more information.

Add-ons

Because of the major redesign and new features in Firefox 4.0, not all add-ons (extensions, themes) that worked in Firefox 3.6 will work in Firefox 4.0. However, the mass majority of ad-on developers have updated their add-ons to work with Firefox 4.0. When you first run Firefox 4.0 after installation it is going to check your add-ons and attempt to update if it is currently incompatible. If an update is not found, the incompatible add-on will be disabled.