The Firefox vulnerability, tracked as CVE-2020-26950, has been described as an issue related to write side effects in MCallGetProperty opcode not being accounted for.

“In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free condition,” Mozilla said in an advisory published on Monday.

The flaw was fixed with the release of Firefox 82.0.3, Firefox ESR 78.4.1 and Thunderbird 78.4.2 just a couple of days after it was disclosed at the 2020 Tianfu Cup International PWN Contest, which took place over the past weekend in China.