Are you opening new tabs by pressing the little button to the right of existing tabs? Assuming that you are...
When you start MC, it decides what to put in each tab based on this setting:
Tools > Options > Startup > Location
I have mine set to "last location" and all my tabs start where I last left them. I do not used locked tabs. I'm not sure what functionality they have. EDIT: I just tested my 3 tab setup by locking all of them, shutting down MC, restarting MC. My tabs appeared as they should with their last contents.
You might also be interested in the Saved Views feature. Ignore the word "views" for a moment. This feature saves the LAYOUT of your MC setup at the moment you save it. So you could have 5 tabs in a particular order, with particular contents. Save that with View > Load/Save View > Save . Later come back to it with the Load selection in that same menu. It's pretty neat for having layouts that you use, or just ones that you want come back to.
Brian.