This does not sound like the exact warning that I get when I close multiple tabs. However I have also changed an accessibility setting that makes this come up.

Try to change the setting from this menu:

1. Open up Options
2. Go to the Tabs sections
3. Uncheck "Warn me when I close multiple tabs"

Restart Firefox and try to close a window with multiple tabs. Did this help?

"Try to change the setting from this menu:"

   "1. Open up Options"

      In my version of firefox it is called "Preferences" ("Options" does not exist)

   "2. Go to the Tabs section(s)"

   Yep, this is the same...

   "3. Uncheck "Warn me when I close multiple tabs""

   In my version of Firefox, the verbage is "Warn me when closing multiple tabs", and yes, it is unchecked.   Originally I discovered that this box remained checked even after I asked the prompt to discontinue warning me, so I manually unchecked this box under Preferences, Tabs.  The box for "Warn me when I close multiple tabs" does remain unchecked now when I go look at it, yet Firefox continues to warn me every time I attempt to close the browser with more than one tab open (won't act as directed...LOL... )

Well, I have found something interesting. I am still not totally sure why Firefox operates this way, but here is what i found. Evidently the programmers felt that if you are going to be allowed to close multiple tabs on exit without being prompted, you are going to have to live with it remembering and reopening your last tabs next time you start Firefox. If you set in Preferences, General, "When Firefox starts:" to "Show my windows and tabs from last time" , it will allow you to close multiple tabs on exit without prompting. However, next time you start Firefox, it (obviously) will start all tabs and windows that were open when you exited. The way I became "clued in to this" was that when I attempted to close multiple tabs, chose "Do not ask me next time", and chose "Save and exit" instead of just "exit", Firefox changed this setting FOR ME, and allowed the browser to close, only to reopen all my tabs on next start. Although this is not really how I want my Firefox to operate, I suppose I can appreciate the logic behind it...

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved

You can look at quit and warn prefs on the about:config page.

• browser.showQuitWarning
• browser.warnOnQuit
• browser.tabs.warnOnClose
• browser.tabs.warnOnCloseOtherTabs

Thanks cor-el,

  Changing to the default theme had no effect.  Starting in Safe mode issue still presented itself (even in safe mode).  

  On the about:config page, I set the browser.showQuitWarning to "false".  Problem solved.  It now closes immediately when I click "x" without prompting.  Thanks again.