Normally, a major release — sometimes called an integer release because the first number of the version changes — comes with significant features that will compel users to upgrade. But the release of, which appears less than three months after Firefox 4.0 hit the Internet (see “ ,” 2 April 2011), lacks any such marquee features. So if Firefox 5.0 doesn’t provide major new features, why is Mozilla bumping the version number so high?
The true story will likely never be known, but there are probably a number of interacting reasons. First, with Microsoft’s Internet Explorer now at version 9, it’s possible Mozilla wanted to increase Firefox’s version number more quickly to make it seem more mature. Then there is Mozilla’s move to a, although I see no inherent reason that should require version number inflation. And lastly, one of the major changes is increased exposure in the interface for Firefox’s support for the  feature — the 5.0 version may be one way for Mozilla to increase pressure on advertising networks and other sites that use behavioral tracking to support the Do Not Track header. (For more about Do Not Track, see “ ,” 14 June 2011.)
Besides the increased exposure of the Do Not Track header preference (it’s in the Privacy pane now), the rest of list little that most users will notice. There are some performance improvements, support for CSS animations, and improved standards support. Spell checking is improved for some locales, WebGL content can no longer load cross-domain textures, and several security issues have been fixed. And, well, that’s about it. (Of course, the  includes nearly 1000 items, but most won’t even be understandable to users, much less noticed by them.)
So again, version 5.0? There’s nothing wrong with the release, and it seems to be working fine for me, but it seems as though it really should have been a 4.1.
Firefox 5.0 is a 27.8 MB download when gotten from Mozilla’s site, though only 9.2 MB when acquired through Firefox 4.0.1’s Check for Updates mechanism (choose Firefox > About Firefox > Check for Updates). As with Firefox 4.0, 5.0 requires Mac OS X 10.5 Leopard or later, but running on an Intel-based Mac; if you’re using a PowerPC-based Mac, you can get, which uses almost exactly the same code as Firefox 5.0.