From Jamie in https://phabricator.services.mozilla.com/D61442:

I think we should probably remove this tabindex. That will probably break a test, but I'd say the test is probably wrong. I don't see any good reason for this to be focusable. That said, this would need to be tested to be certain. Just thought I'd flag this ASAP.

Without tabindex="-1" on .infoBody, these test failures happen:

• toolkit/components/prompts/test/test_modal_prompts.html
  buttons don't focus on OS X, but infoBody does instead - got null, expected "infoBody"

• accessible/tests/mochitest/relations/test_ui_modalprompt.html
  Test timed out.

outline: none;on .infoBody was added along with the tabindex="-1" (still can't remember why the CSS rule was useful, though I think it was platform specific?). It may be removable here too, along with the tabindex.

For more context, the modal can be triggered from alert/prompt/confirm JS functions, and .infoBody is the modal text.

I've dug into Matrix messages from April 23, and I found some context starting from here:

https://matrix.to/#/!jmuErVonajdNMbgdeY:mozilla.org/$kjBXXnDWnT4h_5UXIEdfRDMpr3IVpmb0pA0iEVnEgPc?via=mozilla.org&via=matrix.org&via=igalia.com

Old behaviour was that XUL description elements were apparently focusable, so tabindex=-1 restored that behaviour.