Thought I'd put this here for anybody in the future looking for this.
I had some short stories that I'd saved online in the now sadly-defunct
Mozilla Archive Format. Rather than running an old version of Firefox (or Pale Moon with the
MozArchiver), I decided to take a hand at converting one to ePub.
It was pretty easy! Either you can rename the extension to .zip or use 7-Zip's file manager to crack it open. All the files are in there, under a subfolder.
I was able to extract them all and just import them all into Sigil. Skip the .rdf file; it's just an xml file with the original url and other information. Under index_files you can also skip anything that's not an image or css file.
The downside: Complex pages tend to muck up, so you'll probably need to do extensive editing on those ones to get it to display right.
Hope this helps someone else.