Just as a test, in Opera Mail, goto "Menu -> settings -> import and export -> import mail -> import generic mbox" and import one Thunderbird's INBOX file for one of Thunderbird's pop accounts into a new account.

For example, import "C:\Users\user\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default\Mail\pop.server.com\Inbox" (not Inbox.msf) and see how things come out then.

You can examine the Inbox file with a text editor to see if the problem messages are missing date headers where Opera when then resort to using the import date and possible generating a date header from that.

FYI, Opera's "Import from Thunderbird" wizard is kind of screwy, so it's better to just create the pop accounts in Opera and use the generic mbox importer to import the folders (like Inbox) into their corresponding accounts that you already created in Opera.