Security Vulnerabilities fixed in Firefox ESR 91.6 — Mozilla

A Time-of-Check Time-of-Use bug existed in the Maintenance (Updater) Service that could be abused to grant Users write access to an arbitrary directory. This could have been used to escalate to SYSTEM access.This bug only affects Firefox on Windows. Other operating systems are unaffected.
If a user installed an extension of a particular type, the extension could have auto-updated itself and while doing so, bypass the prompt which grants the new version the new requested permissions.
If a user was convinced to drag and drop an image to their desktop or other folder, the resulting object could have been changed into an executable script which would have run arbitrary code after the user clicked on it.
If a document created a sandboxed iframe without allow-scripts, and subsequently appended an element to the iframe’s document that e.g. had a JavaScript event handler - the event handler would have run despite the iframe’s sandbox.
When importing resources using Web Workers, error messages would distinguish the difference between application/javascript responses and non-script responses. This could have been abused to learn information cross-origin.
Web-accessible extension pages (pages with a moz-extension:// scheme) were not correctly enforcing the frame-ancestors directive when it was used in the Web Extension’s Content Security Policy.
When a worker is shutdown, it was possible to cause script to run late in the lifecycle, at a point after where it should not be possible.
Mozilla developers and community members Paul Adenot and the Mozilla Fuzzing Team reported memory safety bugs present in Firefox 96 and Firefox ESR 91.5. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.