Just close each message as and when you are done with it. Close its tab, just as you would in a browser by clicking its X.

If you right-click a tab, there will be an option to "close other tabs". That will get you back in control.

Or you could set Thunderbird to open messages in a window.

Tools|Options|Display|Advanced

Personally, I don't open messages in windows or tabs. I read them all in the lower half of the main window, below the list of messages.

Zenoi , you are a savior. Thanks. I read in one of the help posts that this tab bar cannot not be fully hidden, but this is a workaround that limitation.

There has been a big push to get things into tabs. So far, we can read messages in them, and we can see the calendar. But the address book and the composition window still appear in separate windows. Some users ask for these to be put into tabs.

I don't think tabs are a good fit to email; each tab is mutually exclusive of all the others. If I wanted to look at the calendar whilst writing a message, tabs mean that I couldn't have both visible at once. Or I might want to look at the address book to copy some contact info. Again, it couldn't be opened alongside the composition window if both were in tabs.

Anyhow, given a deliberate design intention to promote tabs, it's not too surprising that we can't remove the tab bar. But we don't have to use it for opening messages.