The end tag of
<slot> tags is mandatory, but
aboutaddons.html omits them. It uses
<slot />, but since it is not a XHTML document,
/> doesn't count as a closing tag.
The current code results in the expected DOM tree, because the
<slot/> is immediately followed by another closing tag (which causes the HTML parser to insert the missing closing
</slot> tag). However if we were to put some other content after it, then the text would be part of the default slot instead of being rendered after the slot. So let's fix it to avoid surprises in the future.
/> in the file should be replaced with
>. And while we are at, let's also remove the unnecessary
type attributes from the script and stylesheet elements, and fix the alphabetical sorting order of the tests at https://searchfox.org/mozilla-central/rev/d302c3058330a57f238be4062fddea629311ce66/toolkit/mozapps/extensions/test/browser/browser.ini#119-120