1) Type about:customizing<enter> in the address bar. Look for Show / Hide Toolbars at the bottom left.

There is also a Restore Defaults button at the bottom middle.

2) You may have a corrupt xulstore.json file. https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved

I'd already tried the first suggestion and your second suggestion contained a link to Refresh Firefox and that restored History and Menu bar function, thank you so much...

In the interim of posting this question earlier, I discovered that all the folders in my yahoo mail account have disappeared - that would have happened at the same time the issues above occurred. Of course the FF refresh was unable to help with that but now I am concerned as how could all of my email folders have disappeared? These things happened at the same time. Sadly Yahoo has no customer service - is there any chance you'd have any idea what may have happened there? Thank you again, so much!

Note that you should be able to close the Library window via Command+W if it happens again.

You can alternatively rename (or delete) the xulstore.json file in the Firefox profile folder to reset the sizes of windows stored in this file.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox should not have that effect unless you told it to.

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

Whew, what a lot of work! Ok, have given each suggestion a thorough run-thru and the FF issue is resolved, it seems but the Yahoo issue is not, even in safe-mode. But thank you so much for the safe-mode suggestion, unfortunately, it did not correct the email issue. So it is apparently on Yahoo's end [[ cue:lightening and thunder because...well, good luck with non-existent Yahoo solutions, sob...]]

Thank you both for your wonderful and very helpful, help getting FF back together!

Check that you do not run Firefox in Full Screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

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

You can try to use Command+W to open a window that doesn't have a close button.