Could you check to make sure Firefox is not in its "full screen" view:

You should find a button for full screen view on the application menu (the one accessed using the button with the 3 horizontal bars). If you click / unclick that, does it make any difference?

Sometimes the file that stores windows positions and dimensions becomes corrupted and stores nonsensical values. You can rename the file and Firefox will return to default window sizes and positions. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, click the "Show in Finder" button. This should launch a new window listing various files and folders.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit (or Firefox > Quit ?)

Pause while Firefox finishes its cleanup, then rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old.

Start Firefox back up again. Do windows display normally again?

Make sure that you do not run Firefox in full screen mode (Mac: Command+Shift+F).

• https://support.mozilla.org/kb/how-to-use-full-screen

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)

Please bear with me, as I am having a way-too-long day and I will need to respond to your helpful input one piece at a time. The first answer is that before I wrote the question, I had already made sure that I was not in full screen mode.

Now I will go to Users/me/Library/Preferences/org.mozilla.firefox.plist and investigate. I use property list editor to read .plist files, so I should be able to tell pretty quickly whether the preference file makes sense or is corrupted. I'll let you know what I find. Thanks for your help!

On Mac the menu bar is under control of the Mac OS, so if the menu bar is missing then you might be in full screen mode or something in OS X is wrong. I don't think that Firefox could hide the menu bar like is possible on other platforms.

You can also try to repair the disk permissions:

• http://thexlab.com/faqs/repairprocess.html

Or uninstall (i.e. move the Firefox application to the Trash) and reinstall Firefox. You can find the full version of the current Firefox release (40.0.3) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/
• https://support.mozilla.org/kb/Installing+Firefox+on+Mac
• http://kb.mozillazine.org/Installing_Firefox#Mac_OS_X

I'm back to report that I followed jsher2000's suggestion about pulling up the troubleshooting window, locating the 'offending' files, exiting Firefox, renaming the nonsensical files, then restarting the browser. It worked like a charm. I have my menu bar back - and now can forge onwards (presumably upwards), using Firefox.

Oddly, my org.mozilla.plist file appeared normal. It's pretty easy to spot corrupted preference files on a Mac, because they have all kinds of funky characters in them, such as symbols for currencies that have long since been replaced by the Euro.

The solution, it transpires, lay in the Users/Library/Application Support/Firefox/Profiles folder - something I would never have figured out on my own. Many, many thanks!

sara