Was it the last tab?

Hi Robert12, also, could you clarify your reference here:

The blank “Private Browsing” tab would remain open and Firefox would keep running normally.

Normally, Firefox does not mix regular tabs and private tabs in the same window: regular windows have regular tabs and private windows have private tabs.

On your Firefox, is closing the last regular tab in the regular window also closing a separate private window? (That's not normal.)

There is usually what you call “a regular tab” to the left, and a “Private Browsing” tab on the right (see attached screen captures). When I click on the cross button to close the leftmost tab, most of the time Firefox shuts itself down automatically. This wasn’t happening with previous Firefox version. It isn’t a big deal, but it is annoying. I think that a “Private Browsing” tab is something different from a “Private Window”. I never run sites in “Private Windows”, only in regular tabs. But whenever I click on the “+” sign to the right of the tab bar, I get a new “Private Browsing” tab. I can then enter a URL or a search in the address bar (see attached screen capture).

I got into the habit of: when I start Firefox, I drop the first window down to the task bar. I then do whatever I want to do in another window. Once in a while, I unintentionally close the last tab when I didn't want to. From the first window, all I have to do is call the last closed window.

Try to set browser.tabs.closeWindowWithLastTab = false on the about:config page.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

browser.tabs.closeWindowWithLastTab was already set to "false"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

There is usually what you call “a regular tab” to the left, and a “Private Browsing” tab on the right (see attached screen captures). When I click on the cross button to close the leftmost tab, most of the time Firefox shuts itself down automatically. This wasn’t happening with previous Firefox version. It isn’t a big deal, but it is annoying.

I see, I think you are using automatic private browsing mode, i.e., there are no non-private regular windows in your session. Here's how the Options page looks with that configuration:

Still, even with that setting, you should be able to close one of your two tabs without ending your session. I don't know what is causing that problem.

I had not realized this before but it is happening only when Firefox prevented the site in the left tab to open a pop-up window (see attached screen capture). If I click on the cross button to close that left tab, Firefox shuts itself down automatically, along with the blank page being open in the next “Private Browsing” tab.