Mozilla released Firefox 4.0.1 that fixes several security issues as well as stability issues.
Miscellaneous memory safety hazards
Mozilla developers identified and fixed several memory safety bugs in the browser engine used in Firefox and other Mozilla-based products. Some of these bugs showed evidence of memory corruption under certain circumstances, and we presume that with enough effort at least some of these could be exploited to run arbitrary code.
Two crashes that could potentially be exploited to run malicious code were found in the WebGL feature and fixed in Firefox 4.0.1. In addition the WebGLES libraries could potentially be used to bypass a security feature of recent Windows versions. The WebGL feature was introduced in Firefox 4; older versions are not affected by these issues.
XSLT generate-id() function heap address leak
The XSLT generate-id() function returned a string that revealed a specific valid address of an object on the memory heap. It is possible that in some cases this address would be valuable information that could be used by an attacker while exploiting a different memory corruption but, in order to make an exploit more reliable or work around mitigation features in the browser or operating system.