Some HTML elements such as &lt;title&gt; and &lt;textarea&gt; can contain literal angle brackets without treating them as markup. It is possible to pass a literal closing tag to .innerHTML on these elements and subsequent content after that will be parsed as if it were outside the tag. This can lead to XSS if a site does not filter user input as strictly for these elements as it does for other elements. This vulnerability affects Firefox < 69 Thunderbird < 68.1 Thunderbird < 60.9 Firefox ESR < 60.9 and Firefox ESR < 68.1.