browser.bookmarks.file pointed to c:\[relevant beginning of path here; varies by OS]\Mozilla\Firefox\Profiles\[nonsense folder name here].default\bookmarks.html
AND!!! (This made the real diff)
browser.bookmarks.restore_default_bookmarks set to TRUE. Seems maybe the default 'false' (this line wasn't pre-entered but about:config definitions say that's the default) had the effect of making the file *act* like a "Read Only?" None of the bookmarks or folders WERE Read Only but what I mean is, things instead bounced off and recreated themselves under that setting. It wanted to know it had control of being able to restore bookmarks, even tho there is no reason to think mine were corrupted. Now it looks like my bookmarks.html file is staying current, and only the bookmarks.bak file is updating itself more often than it, which is fine.
With this setting combo it seems Fx has also stopped forcing me to have a bookmarkbackups folder, which I didn't until days after install, but which appeared (and would recreate if I deleted it, even adding the dated folders within folders) when I first began tweaking about:config bookmark entries. I think that's more proof it just wants control over backups: now that I gave it that control, it is confident enough not to force a backup folder (or additional bookmark copies) on me.
This has held true through many intentional restarts of Fx today; previously that would create more bkmks each time, spread all over including in the dated folders so they could all have the same name. If any problem presents, such as unwanted recreation when I restart on a different date (i.e., tomorrow), I'll post back. Otherwise, it worked!
Alas, like others I have had additional problems with this version, which cropped up days after install. Have been tweaking about:config for those too (memory, etc.), but can't mess around long... am giving this old PC to a needy friend within days and he knows nothing at all about computers, and will not be able to address anything or switch versions by himself. Too early to tell if they've worked yet, but if not I may have to bag it, rather than leave him with mysterious probs that are likely to get worse.
).