You need to use the keyboard and open the File menu via Alt+F to see a Close Tab menu entry because some menu entries are hidden if you use the mouse.

Can you close a tab vi Ctrl+W?

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

All right, I was able to get the "Close Window" option to appear. However, the window still would not close, or drag to another window. I tried Fine->Close Window, Ctrl-W, and clicking the tab.

The problem seems to begin only after playing Farmville on Facebook, so I assume there is something on that website that interferes with a Firefox function.

Did you try Safe mode to check if an extension is causing that problem?

Yes. I used Safe mode. The problem only occurs when I am using Facebook, apparently; some scripts loaded by Facebook seem to interfere with one or more scripts in "tabbrowser.xml".

Specifically, the error is reported to be in this line:

Script: chrome://browser/content/tabbrowser.xml:1644

I can often stop the scripts from Facebook, and then the tabs can still close; but if I stop TabBrowser.xml, tabs will no longer close. Unfortunately, when the FB scripts crash, I usually have little choice but to close the TabBrowser.xml script.

This now happens constantly while I use Firefox on Facebook. It no longer happens unless I am using Facebook, and is apparently unrelated to whether I play Farmville.