I must have bumped something by accident.

My top menu bar and display of open tabs is gone. I have to move my mouse to the top of my screen to see it.

What did I do that make my browser display this way?
And
What do I need to do to get it back to a static display of the menu items and tabs?

Thanks!

Press the "Alt" to view a hidden Menu bar then select View > Toolbars

View > Toolbars for starters. (alt +V if you can't see it)

Sounds like you have made you tab bar auto hide, possibly in an extension.
What have you got installed ?

I closed FF and then reopened it and everything is OK now.

Not only was my tab bar hideen but the Menu Bar, Navigation Toolbar and Bookmarks Toolbar were also hidden, so I can only presume that I somehow bumped the right combination on my keyboard that hid them all.

Thankfully it's fixed now. If it happens in the future, is there a keyboard shortcut that will turn them all back on?

Perfect, thanks!