Firefox 4 will be the next major release of Firefox that Mozilla scheduled to release tentatively sometime before the end of 2010. Remember that along the way there are builds, the alpha, Firefox 4 beta and RC versions, which you can test if you'd prefer being advanced.
Major features of Firefox 4

Firefox 4 is envisioned and described as a fast, powerful, and empowering browser.
Mozilla has turned the supposed Firefox 3.7 into Firefox 3.6.4, the final version of which has been released. It was motivated by the out-of-process plugins feature.
That is why Mozilla will be releasing Firefox 4 beta starting late June. Firefox 4 Release Candidate (RC) will be shipped in October and Firefox 4 Final at the end of this year.

If you want to download Firefox 4 beta starting this June, click this link.
Major features of Firefox 4
- HTML5 - the major revision of HTML (HyperText Markup Language) aimed to reduce the need for proprietary plug-ins such as Adobe Flash and Microsoft Silverlight. HTML5 is widely supported by companies such as Apple (so that videos on major websites particularly news sites can be viewed on the iPad), YouTube, Scribd and many more.
- WebM - an open-source, royalty-free video format which will be available to everyone. This already is being implemented on a test phase at YouTube. A CNET news report last May 19 says "Google, Mozilla and Opera are all adding WebM support to their browsers and all videos that are 720p or larger uploaded to YouTube after May 19th will be be encoded in WebM..."
Current videos in Flash format has the <object> element. I'm sure that webmasters and bloggers are familiar with this. In WebM, it will be changed into <video>. Read this post for the details. - New Theme - Mozilla has released several Firefox 4 Windows theme mockups to give us a picture of what the future browser will look like. Here's a mockup showing a Windows 7 Aero version with tab-on-top, in large button mode.
Firefox 4 is envisioned and described as a fast, powerful, and empowering browser.
Mozilla has turned the supposed Firefox 3.7 into Firefox 3.6.4, the final version of which has been released. It was motivated by the out-of-process plugins feature.
That is why Mozilla will be releasing Firefox 4 beta starting late June. Firefox 4 Release Candidate (RC) will be shipped in October and Firefox 4 Final at the end of this year.
If you want to download Firefox 4 beta starting this June, click this link.