You can go to about:config search for browser.tabs.onTop change its value to false.

• about:config

Also you can customize or restore your toolbars:

• Customize toolbars

Also:

• Firefox in safe mode
• Reset Firefox

Thanks Diego, that's a useful reminder of the here-be-dragons way of changing the setting. I've done that and it works. However, I've not marked that as solving my problem. I want to know what happened to menu items in the main menu for it. Also, I appear to be missing a menu item (presumably it was under View as well) for toggling the status bar. I haven't got one right now. I might want it back. I could probably find it in the config editor, too. I just don't like things not working right, and it makes me wonder what else could be missing in the menus.

Customize toolbars isn't particularly related to the issue at all, as far as I can see.

I haven't tried starting FF in safe mode. Thanks for the tip. I've not had much to deal with like this - to the FF team's credit, it works pretty well for me - so I haven't got into the advanced stuff.

Reset Firefox - don't get me started. I realise this is partly my fault, but I already tried that, tired and fed up and rushing things, and didn't back up. I thought it might lose my current tabs and groups. I thought it might lose my bookmarks. Probably won't. It's Firefox, after all. I never thought it would lose my extensions! Afterwards, it's fairly likely to - it resets it to default ... but then, it KEPT my bookmarks and tabs and groups! I was so sick. I've spent years getting the addons I want to make FF work just the way I want it. Some of them have advanced settings I had to tweak. It would take ages to get it all set up again. There should be more info on the help page about what will be kept and lost. But I intended to post about that - it's only because you mentioned it here that I'm going on about it.

Finally, after deciding not to change to Chrome (Evil Empire), downloading firefox again and preparing to reinstall it (I might as well see if I can get the full menus back!) - I saw the reset had put my old profile in a new folder on my desktop. Hey, how about someone mentioning that? Jees. After a bit more reading, I copied that back into the profiles folder, it wasn't immediately recognised, but I created a new profile, named it "Restored Profile" and then gave it the location on disc of the reinstated folder, and bingo, I was back with my AdBlock and other extensions. Whew.

The status bar is no longer used in current Firefox releases and is replaced by the Add-on Bar (View > Toolbars).

Current Firefox versions only show the "Tabs on Top" menu entry in the "View > Toolbars" and "Firefox > Options" menu and in toolbar context menus if the tabs aren't in the default position on top.

If the tabs are on top and the menu entry isn't available and you want to move the tabs below the navigation toolbar then you need to toggle the browser.tabs.onTop pref to false on the about:config page. A Firefox restart is necessary to update the menu entry to either show or remove it.

Note that this pref will no longer have effect when the Australis code lands on the release channel (that will be in Firefox 29.0).

• http://www.ghacks.net/2014/01/31/mozilla-decides-uplift-australis-firefox-aurora/

Some menu entries in the main menu bar are hidden if you use the mouse and only appear if you use the keyboard to open the menu.

Some entries in the View and History menu have been removed to clean up the user interface because studies showed that these were low usage items.
On Windows and Linux these were already hidden when you used to mouse to access the menu bar and only appeared when you used the keyboard.
You can click the toolbar Home button (far right on Navigation Toolbar) or use the keyboard (Alt/Option + Home).
You can find Back, Forward, and Reload in the right-click context menu in the browsing area or use the buttons on the Navigation Toolbar.

See also "Help > Keyboard Shortcuts"

• https://support.mozilla.org/kb/Keyboard+shortcuts

Thanks cor-el. That'll be it. Very well thought through...I call that solved.

I will never have restarted the program before with tabs on top, as I never liked it, until using TabGroups Manager. Then, with it set via the menu, I removed TGM and it restarted in that condition. Firefox then removes "Tabs on Top" from the menu (because they are on top). This may not be the best place to say this, but that's pretty unhelpful, IMHO, to reduce the clutter on the View > Toolbars menu, with about half a dozen items on it.

I have tested it again now. If I restart with tabs below the address bar, it puts the item back in the menu. If I restart with them on top, it removes it. Hence, if a user wants to try it, or uses it temporarily for more space on the screen (in preference to full screen mode), they can't put it back without going into the config settings, AFAICS. It's not on the keyboard-activated menu (I don't see any difference), nor in other option dialogs, AFAIK. Since there's that "Here be dragons" warning on the about:config screen, it seems rather odd, dangerous and unhelpful. Besides, for anyone who likes things to stay put on menus (me), it prompts queries like this conversation, and nearly cost me my Firefox addons. A lesser mortal might even have defected to the dark side. ;)

Thanks again, both of you - I've learned a lot from this, not least to back up my profile to my external drive every now and then.