I've solved the problem (see below) but I left this here in case some other fossil is still using an "no longer supported by your OS" version of Firefox and needs to patch things up.
I've looked all over and I can't find a way to import a bookmarks backup file in 52.9 ESR on Windows XP. Instead of the |||\ "library" button, there's just a panel with several buttons, none of which lead to the Bookmarks / All Bookmarks page with the Import/export menu that's found on my later versions.
I can find the Profile, and when I look there I find a bunch of backups (.jsonlz4). I've added one freshly exported from another Firefox. Still, when I open the Bookmarks menu, what I get is an ugly list of bookmarks that I cleaned up in the "nice" version by deleting useless ones and creating some new folders. I deleted all of the other backups in hopes that it would choose this one, but apparently not. Or maybe for some reason my changes before I exported the bookmarks to the backup file aren't in there.
The reason for all of this nonsense is that, for some unknown reason, the bookmarks on my primary computer got messed up and the 52.9 version (on my laptop) had Sync turned on, so it promptly inherited the messy bookmarks. And to add insult to injury, the primary computer's Firefox synchronized to the laptop and messed up the bookmarks again - which I was able to correct by importing the latest backup.
Does Mozbackup still work? That's what I used to use when setting up a new computer. Or is there a place where I can put a copy of the HTML backup of the good bookmark set in the 52.9 version?
This is driving me nuts and I've wasted entirely too much time on it already. I know I should get a new computer with a newer OS, but I'm making do with everything else I need on this computer except a web browser that will load some of the pages that Explorer 8 won't.
I just tried another experiment that might explain things, but poses another question. I created an HTML backup and imported that into Explorer. I had the same mess of bookmarks. I cleaned up the bookmarks by deleting and dragging links into new or other folders, and I got what I was looking for. Is it possible that what I see on the screen when I click on Bookmarks isn't really what's in the exported bookmarks files? If not, how to I do what I'm trying to do, which is make the bookmarks look the same on two different computers with two different versions or Windows and two different versions of Firefox? Or is that asking too much?