The menu bar is a toolbar. You turn toolbars on and off under View-Toolbars.

You have the choice of opening messages in a new window or a tab. That setting is under Advanced-Reading and Display

It already is selected. The problem is that it only is visible if I hold my cursor on it; if the cursor is moved then the menu bar disappears.

I have that set to open in an existing message window.

1. see http://lifehacker.com/5280456/hide-your-mac-menu-bar-and-dock-for-a-cleaner-desktop and adjuect you osx setting if Thunderbird are not working it must be operating system. (I assume you have rebooted your computer. Mac users have some unrealistic idea that a computer does not need rebooting, even when it starts doing weird things. unfortunately they are wrong.)

2. if your messages are opening in Tabs, that is as far from tabs as you can get. Thunderbird is a tabbed application, like it or not.

Yes, rebooted the iMac but it still was happening. However, today the Menu Bar was back; weird. BTW, that link you posted is about 5-6 years old. It was discussing Mac OS 10.5.x, while I have Mac OS 10.7.x, the current is Mac OS 10.9.x, and this fall Mac OS 10.10.x will be released.

Hmmm, so Mozilla is degenerating T-bird like it did Firefox with the release of "Chromefox 29". Looks like after umpteen years of using Mozilla products in lieu of Apple's, I'll have to start using the latter.

SeaMonkey is not degenerating. You can use that for both browsing and email and newsgroups.

Use what you like romad, but personally I have come to like the Australis theme in Thunderbird and I downloaded Firefox alpha when I heard that after a year or fussing and fiddling they have finally moved forward.

Seriously I have no idea what your talking about though. You carrying on like Thunderbird suddenly changed. Australis has been there since Version 17 tabs since Version 3

As far as your minority operating system is concerned. I have never used it and have no desire to try and decipher what apple did to a perfectly good copy of BSD. If the product changes so radically on point releases that information a couple of years old is worthless, I am sorry. I am used to point releases being minor changes and whole numbers containing the big stuff. However as Thunderbird does not have a feature of the kind you describe, but apple users have been complaining of similar for years in a number of different applications I suggest you ask apple how to fix OSX so it does not do it.

Does Thunderbird still have the setting in about:config, "mail.tabs.autoHide"? If so, you could set that to true, and not see the tab bar if you use only one tab.

In Mac OS 10.6, the only menubar setting I have in System Preferences is in "Desktop and Screen Saver", and that just makes the menubar translucent or not. Maybe it is different in Mac OS 10.7?

Are you using full-screen mode with Thunderbird? The only reference I could find for autohiding the menubar was one about full-screen. If you want a large window, you could use maximize instead of full-screen.

Nope, I don't use Full Screen mode. Hmmm, maybe that is what happened: some setting got corrupted and put T-bird into permanent Full Screen mode. However, I thought that logging out or rebooting canceled that mode but mine stayed that way for 2-3 days (none of my other open applications were in Full Screen mode). No I don't want the Menu Bar hidden