When reading from areas partially or fully outside the source resource with WebGL's <code>copyTexSubImage</code> method, the specification requires the returned values be zero. Previously, this memory was uninitialized, leading to potentially sensitive data disclosure. This vulnerability affects Thunderbird < 68.7.0, Firefox ESR < 68.7, and Firefox < 75.
https://www.mozilla.org/security/advisories/mfsa2020-14/
https://www.mozilla.org/security/advisories/mfsa2020-13/
https://www.mozilla.org/security/advisories/mfsa2020-12/