What's new in Firefox 3.6.18, Mozila only mentions:
Fixed several security issues.
Fixed several stability issues.
And the security issues were:
MFSA 2011-24 Cookie isolation error
MFSA 2011-23 Multiple dangling pointer vulnerabilities
MFSA 2011-22 Integer overflow and arbitrary code execution in Array.reduceRight()
MFSA 2011-21 Memory corruption due to multipart/x-mixed-replace images
MFSA 2011-20 Use-after-free vulnerability when viewing XUL document with script disabled
MFSA 2011-19 Miscellaneous memory safety hazards (rv:3.0/184.108.40.206)
Mozilla Firefox 3.6.18 is available here: