This is also a permafailure on macOS when running the test standalone on desktop Firefox:

./mach mochitest dom/html/test/test_bug430351.html
…
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="parent"><iframe></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="-1"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="parent"><iframe tabindex="-1"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="0"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="parent"><iframe tabindex="0"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="0" disabled=""></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="parent"><iframe tabindex="0" disabled=""></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="1"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="parent"><iframe tabindex="1"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" disabled=""></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="parent"><iframe disabled=""></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" contenteditable="true"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe contenteditable="true"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="-1"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe tabindex="-1"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="0"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe tabindex="0"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="0" disabled=""></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe tabindex="0" disabled=""></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" tabindex="1"></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe tabindex="1"></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]
FAIL <iframe xmlns="http://www.w3.org/1999/xhtml" disabled=""></iframe> in <div xmlns="http://www.w3.org/1999/xhtml" id="editableParent" contenteditable="true"><iframe disabled=""></iframe></div> should be focusable - got [object HTMLBodyElement], expected [object HTMLIFrameElement]

Perhaps this bug should move to a non-GV component or do you want me to file a separate bug for that?

(In reply to Matthew N. [:MattN] (PM me if requests are blocking you) from comment #5)

This is also a permafailure on macOS when running the test standalone on desktop Firefox:
...
Perhaps this bug should move to a non-GV component or do you want me to file a separate bug for that?

I'll move this bug to the Core::DOM component.

Sounds like this might be a focus manager bug?

It looks like this test is doing something similar to:

parent.innerHTML = "<iframe tabindex=0></iframe>";
parent.firstChild.focus()

But the iframe isn't really ready yet so it isn't focusable yet.

Interestingly, there is a line in the test that gets the element.contentDocument in-between setting the content and focusing; this has the effect of initializing the iframe, but it filters out iframes with no src. Removing this filter makes the test pass fine. This is at https://searchfox.org/mozilla-central/rev/92d11a33250a8e368c8ca3e962e15ca67117f765/dom/html/test/test_bug430351.html#464

https://hg.mozilla.org/mozilla-central/rev/65f7904484c8

Is this something you may want to uplift to 67 beta?

This is test-only - let's just take it.

https://hg.mozilla.org/releases/mozilla-beta/rev/77f9f8131845