The Firefox and Firefox Focus UI for the Android custom tab feature only showed the "site" that was loaded, not the full hostname. User supplied content hosted on a subdomain of a site could have been used to fool a user into thinking it was content from a different subdomain of that site. This vulnerability was fixed in Firefox 144.
Links in a sandboxed iframe could open an external app on Android without the required "allow-" permission. This vulnerability was fixed in Firefox 144 and Thunderbird 144.
If a user saved a response from the Network tab in Devtools using the Save As context menu option, that file may not have been saved with the `.download` file extension. This could have led to the user inadvertently running a malicious executable. This vulnerability was fixed in Firefox 140 and Thunderbird 140.
Memory safety bugs present in Firefox 139 and Thunderbird 139. 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. This vulnerability was fixed in Firefox 140 and Thunderbird 140.