According to an advisory from the open-source group, 8 of the 10 vulnerabilities are rated "critical," meaning that they can be used to run attacker code and install software, requiring no user interaction beyond normal browsing.
Here's a glimpse of the critical issues:
Mozilla identified and fixed several memory safety bugs in the browser engine used in Firefox 4, Firefox 5 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.
These include a WebGL crash, a JavaScript crash, a crash in the Ogg reader, memory safety issues and unsigned scripts. These all affected Firefox 4 and 5.
Mozilla also credited researcher Michael Jordon of Context IS with reporting a pair of critical issues -- that an overly long shader program could cause a buffer overrun and crash in a string class used to store the shader source code; and a potentially exploitable heap overflow in the ANGLE library used by Mozilla's WebGL implementation.
Some additional security problems fixed: