re:''changing my accounts from IMAP to POP. '' I presume you did not simply swap the account server settings from imap to pop, but you did create a new mail account for that email address and set up as Pop. re :''So my questions are, why are these e-mails appearing in the search results window yet I cannot make them open,'' Because your search database is not up to date and the emails are blank because they do not exist. Suggest you rebuild your global database. https://support.mozilla.org/en-US/kb/rebuilding-global-database Why are the emails disappearing? Possible reason: Do you have any Anti-Virus product scanning Thunderbird folders on startup or on opening files or during a scheduled scan etc ? If yes, then it is possible those mbox files containing the emails are being scanned and quarrantined and if auto 'fixed' by the Anti-Virus then they could be completely deleted. This would leave the global database and index files still intact show a list or search result but unable to actually show you the email content because the Anti-Virus product removed the actual emails. Stop Anti-Virus product from having access to scan your Thunderbird folder and any file therein. (The one in the 'Appdata/Roaming' location). When you are 'moving' a load of emails to another folder, are you sure at that point you are actually moving emails or just headers of emails that have already been lost at some point? When moving emails from one folder to another, they are being moved between mbox files, but is some program scanning the files that have been opened? re:''And one final aside: Thunderbird routinely asks me if I want to compress my folders. It seems to me that it is usually just after letting it perform this operation that I find e-mails vanishing on me. '' When you delete a message or move it from one folder to another, the mbox file does not automatically get smaller. This is because the original message is simply marked for deletion and hidden from view. It is not physically removed until you "compact" the folder. You may see the deleted email appear in the 'Trash' folder, but in reality, it has as yet not been fully deleted from the original mbox file. This means that if you need to retrieve the email, it is easily done. For a 'moved' email, the compacting action would occur on the original folder that previously contained the email and not on the 'new' folder containing the actual moved email as it is unlikely anything would have been deleted in that folder after moving emails into it. Which is why I'm wondering if you actually moved an email or a header? Please confirm: * Your Account Settings for the POP account are downloading full emails and not just headers. * You have excluded all Thunderbird folders and files from being scanned by Anti-Virus. Compacting (not compressing - there is a big difference) is a means of removing the emails that have been 'marked as deleted' from the mbox file. It cleans up the mbox file and reduces it's size. I would recommend manually compacting any folder that has had several emails deleted or moved. This usually effects the following folders, Inbox, Drafts, Junk, more than other folders. I delete and move several emails every day and so perform compacting of specific folders as a matter of housekeeping at the end of each day. For Compacting: There is a setting in Account Settings to do this automatically and I suspect that is the pop up you are seeing. It would not be helpful if the Anti-Virus product is scanning at the same time. You could try switching off the automatic compact and only performing daily manual compact on specific folders, allowing time for each folder to be compacted prior to doing it on the next.