I appreciate the amount of info you've provided, but what I find missing is the circumstances under which this happens… Is it when you receive mail, when you try to move messages to some specific folder, or what? And what type of account is this (POP, IMAP)?

Sorry not to have included that... but Yahoo is my email 'provider', and I can access my messages on their server via mail.yahoo.com and the messages are there. I've been downloading them from Yahoo to Thunderbird for more than a year without problem... and when I tried to do that a few days ago, the "Can't write..." error popped up after it logged into my account and began downloading. This is a POP account. I am able to send mail from Thunderbird, just not able to get it from Yahoo. No other 'events' (upgrades or problems) have happened recently. Sure appreciate any suggestions you might have.

OK, so this is the Inbox of your POP account that cannot be written to when checking for new mail…

What about moving mail out of there? Do you keep all your mail stored in a single huge Inbox? Try creating one or more folders under Local Folders and moving some or all of your mail there. Do you get any error messages as a result of doing that?

If not, move all your mail out of Inbox and into other folders, then right click on Inbox and choose Compact. Do you get any errors when trying to do that? If not, and you may successfully compact it, does that solve the problem?

I am able to move messages from Inbox to other folders, and have deleted many messages and compacted (all mailboxes with no errors). Then I moved everything out of the inbox and compacted... still get "unable to write the email to the mailbox when trying to d/l from mail.yahoo.com.

Start Thunderbird in Troubleshoot Mode and see whether the problem happens there as well. The following article explains how to do that and why it can be useful to help identify the cause of a problem:

https://support.mozilla.org/kb/troubleshoot-mode-thunderbird

I expect the problem to persist even in Troubleshoot Mode, in which case I'll give you instructions to remove the files associated with this folder from your profile folder in the filesystem, to make Thunderbird create them again, but it would be desirable to try to isolate and identify the issue that's causing this if we can, which according to what you said you've tried, cannot be the antivirus…

Well... have discovered it is a Windows Problem... don't yet know exactly what... but started the computer in Safe Mode with Networking... and I was able to d/l and write to the Inbox. I'm adding items back into my 'startup list' one or two at a time... but have not yet discovered the offending program or service. Thanks so much for your suggestions and help.