Well, I went into the profile folder. There was no pref_js file but there was a pref.
js one. Double-clicking on it came up with a message suggesting that it was incorrect in some way so I added an "OLD" to the name, closed Thunderbird and restarted my computer. It dutifully created a new pref.
js file at the bottom of the list of the profile folder's contents - so far, so good.
I went back to account preferences and still it does the same - appear to accept changes to the junk rules but then have forgotten those changes when I revisit each account five seconds later. I went back to the profile folder and the new pref.
js file comes up as faulty when I double-click on it. While I was there I deleted the pref.jsOLD file but it made no difference.
I wonder if I can reinstall Outlook Express..... (only joking!)