The update fixes 50-odd bugs in total, amusingly including three fixes listed as specific to OS/2. Ironically, the latest official release of the OS/2 port of Firefox, dubbed Warpzilla, hasn't yet reached version 4 - it's still back at version 3.6.8.
The release notes for Firefox 4.0.1 are hard to find from the main Mozilla.com page. (Browsing to Firefox.com doesn't help, as this just redirects to the Mozilla page.) But if you know where to look, you'll find that two critical security advisories are fixed in the 4.0.1 release.
MFSA2011-12 deals with memory corruption bugs in the browser engine itself; Mozilla experts officially opined that "with enough effort at least some of these could be exploited to run arbitrary code". MFSA2011-17 deals with "two crashes that could potentially be exploited to run malicious code" in a graphics library called WebGLES, used by Firefox.
Because the 4.0.1 update addresses vulnerabilities that are considered remotely exploitable, we advise you to apply this update without delay.
The previous version, Firefox 3.6, also gets an update, moving to 3.6.17. This update also squashes some critical bugs, including the MFSA2011-12 memory corruption vulnerability affecting Firefox 4.
Two other critical vulnerabilities which don't affect version 4 are fixed.
MFSA2011-13 deals with various "dangling pointer" bugs (a dangling pointer is a programming mistake in which a memory reference remains in use after the memory it points to has been returned to the operating system for re-use). MFSA2011-15 deals with a privilege escalation bug in the Java Embedding Plugin.
The MFSA2011-15 vulnerability is specific to the Mac OS X version of Firefox. Apple users who imagine themselves invulnerable simply by virtue of their choice of operating system, please take note!
Somewhat confusingly, the Thunderbird release notes don't list any critical vulnerabilities fixed in this version, but the MFSA2011-12 advisory specifically states that the bugs it covers are "fixed in Thunderbird 3.0.10".
If you're a Thunderbird user, we advise you, too, to update as soon as you can.