I'm not seeing how to lock this from view using the master password.

The master password is not supposed to protect the contents of the profile. It only protects your saved account passwords.

For other options to protect the profile see http://kb.mozillazine.org/Protect_the_profiles_contents

Uh, no it doesn't only protect saved account passwords, it also allows my emails to be downloaded, as I stated in my original post. I can't download new emails without entering the master password, yet people using my laptop can still read downloaded emails. This doesn't make much sense.

Are you sure you know enough about how this works? I'm not seeing an option to make this behave sensibly.

Uh, yes. Here educate yourself before you dispute the correct answer. https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-master-password

Next time you are asked for the Master Password hit cancel a few times and download your email. How protected is that now?

christ1, you literally said the master password "only protects your saved account passwords", which is not true. It also prevents retrieving new messages. Your link doesn't disprove that.

However, neither of those things are the issue I'm asking about.

I'm asking how to hide messages right after Thunderbird is first opened but the master password hasn't been verified yet. This means that right now if someone else is using my laptop and opens Thunderbird, they can cancel the master password prompt and view my already-downloaded messages.

Your link only addresses this by saying Thunderbird can hide the messages if I make multiple Windows user accounts. That isn't really a Thunderbird solution, is it? It's more like a Windows work-around.

I could just have someone else use the Guest account, but that isn't necessarily feasible all of the time either. So there doesn't appear to be a Thunderbird solution, ie, just having a checkbox setting like "Hide downloaded messages until master password is verified".

I'll just have to weigh forcing anyone else to use the Guest account or their own account, but it really makes no sense to me that the master password lock doesn't prevent downloaded messages from being shown.

So, your Word documents, Excel spreadsheets, your photographs, your music collection, your browsing history - how do you keep these private? And how do you stop others from messing with your settings?

You make use of User Accounts in the operating system. Each user has their own private space. User Accounts are designed specifically to allow multiple users to share a machine and yet have privacy. It is not a "workaround", it is by design. Why should an email client work any differently?

The Startup Master addon may appeal to you. It will not show Thunderbird until the correct master password is entered. Be aware that it is largely cosmetic. Anyone with access to your filesystem can access your mailstore folders and files, and a savvy user could disable it by starting Thunderbird in Safe Mode. I use it to avoid multiple password prompts.

And you will have to remember to close Thunderbird if and when you leave the machine for others to use, whereas a password protected screen saver and User Accounts for your guest users will address this.

It doesn't make any sense to me that you allow others to use your login and then complain about lack of privacy.

christ1, you literally said the master password "only protects your saved account passwords", which is not true.

Sounds like you're an expert. http://kb.mozillazine.org/Master_password

Your link only addresses this by saying Thunderbird can hide the messages if I make multiple Windows user accounts.

I'd suggest you re-read the entire article.