In effect the ' move' command will remove it off the server, so there is also a delete command in the background and I think this issue is related to that 'delete' command and something weird going on with default browser settings.

Try this: Exit Thunderbird. Set the default browser to the one that came with Windows eg: Internet Explorer Start Thunderbird Test 'move' email to a folder in the 'Local Folders' mail account.

Then Exit Thunderbird and reset default to Firefox Start Thunderbird Test 'move' email to a folder in the 'Local Folders' mail account.

Please report back on whether this resetting browser method worked or not.

General comment: I must admit, I always use the 'Copy to' =- not 'move' when needing to keep a copy in 'Local folders' because if something goes wrong in the 'move' you cannot repeat because it is deleted off the server. Recently, had someone who did same as you and lost emails because he did not realise he had only downloaded headers. Another tried to perform move and Anti-virus screwed up email as file was scanned. When dealing with Imap accounts any instruction will involve a load of jibber jabber with server and can also involve other programs like AntiVirus. But, this info is only from my perspective and the choice is ultimately yours.

I appreciate the suggestion, but no, it didn't work. When I move a message from the Inbox to a local folder, the next message in the Inbox still pops open in whichever Web browser is the default browser.

But in the course of testing the suggestion, I did discover something else: This only happens when the message I'm moving is open for reading. In other words, I have the message open, and I right-click in it, choose Move To | Local Folders | [name of folder]. But if the message is NOT open and I move it from the list of Inbox messages to a local folder, it moves normally and the Web browser does not open. Perhaps that will give a clue to what's going on.

re :This only happens when the message I'm moving is open for reading.

Well spotted. Is it open and reading in the 'Message Pane ' or open in new 'Tab' or a 'window' ? Can you do some tests on each of those 'open' options and tell me if it affects all three or which ones work ok and which ones get that open in browser?

Can you tell me what setting you have here: Menu app icon > Options > General scroll to 'Reading and Display' section Open new message - what is selected? Is this checkbox selected 'Close message window/tab on move or delete' ?

I always read Thunderbird messages in a new window.

I normally don't have the Message Pane visible, but I turned it on for this experiment. (1) While reading a message from my IMAP Inbox in the Message Pane, if I move it to a local folder, the next IMAP Inbox message opens up in the same Message Pane, which is the behavior I would expect. (2) If I open a message from my IMAP Inbox in a new tab, and then move that message to a local folder, nothing unusual happens. It works as expected. (3) The option I regularly use, reading each message in a new window -- well, you know what happens then. When I move an open message from my IMAP Inbox to a local folder, the default Web browser opens up, displaying the next message from the IMAP Inbox.

In the area of Options you asked me to visit, the selection for "Open messages in:" is "A new message window." The box for "Close message window/tab on move or delete" is checked.

Thanks again for your interest.

Do you see the same issue if you open an email to read in imap folder and then choose to delete it instead of move it?

No. When I delete a message, no Web browser opens.

Do you use an addon called 'Thunderbird Conversations' ? This addon has been reported as causing some odd behaviour with opening email in browser. Please restart Thunderbird in SAfe Mode and see if this now works ok.

I don't use that add-on. I had one other add-on, which was incompatible with the current version of Thunderbird and which I have now deleted, but the behavior continues.

When I restart Thunderbird in Safe Mode, the behavior does not occur.

if you click the reset toolbars and menus in the safe mode startup and restart do you still see this?

There is also the possibility that your anti virus is injecting an addon into the running Thunderbird process. This is especially so for spam and scam tools. Safe mode prevents these injections by disabling the addon API on the program. If you restart you computer in safe mode with networking and then Thunderbird in normal mode that might point us somewhere as the anti virus will be disabled in Windows safe mode. For Microsoft's Instructions on starting window 10 in safe mode click here.

It apparently isn't the antivirus; I restarted Windows in Safe Mode with Networking, opened Thunderbird, opened a message in my IMAP Inbox, and moved it to a local folder, whereupon a Web browser opened up with the next Inbox message in it.

I haven't tried your other suggestion about resetting toolbars and menus, because I don't want to lose the customizations I've made to Thunderbird over the many years I've been using it. I'd sooner live with the occasional unwanted Web browser launch.

Thanks for your interest.

Also, based on Toad-Hall's original response to me, I've found that there is a work-around: If I COPY the message from the IMAP Inbox to the local folder, there's no unwanted Web browser launch. Of course, I then have to take the extra step of manually deleting the message from my IMAP Inbox. But at least this way I stay in Thunderbird rather than having to close an unwanted browser window.