There are two options that you could use: '''Option 1: Move messages in batches''' Select and Copy messages from the Local Folders Inbox to the online account's Inbox, then delete the originals from the Local Folders Inbox. If you have thousands of messages, don't copy all of them at once. There is a general consensus to not select more than 2000 messages for copying at a time, to prevent Thunderbird from freezing/hanging when trying to process too many messages. I don't know if this is still a problem with the latest release v115 because I haven't had to do this in a long while. Copy them in smaller batches until all messages have been copied over and none is left in the original location. '''Option 2: Use a Global Inbox''' Thunderbird offers the option to configure POP accounts to deliver messages into one Inbox folder. For example, you can change your account's settings so that it delivers messages into the Local Folders Inbox instead of its own Inbox folder. The messages that already exist in the Local Folders Inbox will not be affected in any way. More info is available here [[Unify your POP email accounts with a global inbox]]