Use-after-free vulnerability in the nsHostResolver::ConditionallyRefreshRecord function in libxul.so in Mozilla Firefox before 29.0, Firefox ESR 24.x before 24.5, Thunderbird before 24.5, and SeaMonkey before 2.26 allows remote attackers to execute arbitrary code or cause a denial of service (heap memory corruption) via vectors related to host resolution.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.