If you open Tools/Options/Advanced/General/Config. editor and paste signon.rememberSignons in the Search box, you should find it has the value true, not false:

http://kb.mozillazine.org/Password_not_remembered_%28Thunderbird%29#No_checkbox_to_remember_passwords

I quoted the line from my prefs.js incorrectly in my problem statement... it already has "true" in that line and the checkbox does not show. This is why I am perplexed.

Another user recently had this problem, so it might be worth looking through that discussion in case any of it's relevant:

https://support.mozilla.org/en-US/questions/994267?esab=a&s=&r=31&as=s

You could set up a new profile with Profile Manager, add an account, and see if the problem remains.

http://kb.mozillazine.org/Profile_manager#Creating_a_new_profile

Thanks for the reply. I went through all of the steps taken in the other post you suggested... sounds like an "instant replay" of my efforts. I've exhausted all the fixes suggested in the TB FAQs. I'll give your other suggestion about setting up another profile a try. I don't use any registry cleaners, like the fellow in the other post. More later...

Thanks, sfhowes! I defined a new profile and everything seems to work correctly... remembers passwords, etc. Apparently, something in my original profile got corrupted but, whatever it was, it didn't show up in any of the expected places. All it cost me was redefining my email accounts... not a bad price to pay. Thanks again... Warren

I'm glad it's now working. If you need to transfer any data from the old profile to the new one, instructions are given here:

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird