Enough things use it, but we have zero docs on it, so it's a pretty big black box to add-on authors (and new contributors).
https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/NsIVariant It looks like solved already. If so, please close this bug.
There's a page, but there's not much actual text on the page. It needs to be filled out with actual explanations of what everything does.