Hi ajith1, I think the behavior is still built-in because you can still manipulate the preferences using the Configuration Editor:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste warnon and pause while the list is filtered

Here you'll find numerous preferences such as:

• browser.tabs.warnOnClose (default is true)
• browser.tabs.warnOnCloseOtherTabs (default is true)

However, a few versions ago, Firefox changed from warning for two or more tabs closing to warning when more than 25 tabs are closing. The reason it's 25 is that Firefox can "undo" the closing of up to 25 tabs (by default). So if you accidentally close a large number of tabs, you can restore them one-by-one using any of these methods:

• right-click tab > Undo Close Tab
• Ctrl+Shift+t
• "3-bar" menu button > Library > History > Recently Closed Tabs
• Library toolbar button > History > Recently Closed Tabs
• (menu bar) History > Recently Closed Tabs

Was that what you were looking for, or one of the other warnings?

If you set Firefox to automatically restore your session, the warning for closing out your session is near the beginning of the Options page, or set browser.sessionstore.warnOnQuit to true.

Note that the items in Options/Preferences are only present when the related prefs are modified and do not have their default value. When the warn prefs have the default value then Firefox hides the warn items in Options/Preferences.

Thanks for both your replies, they were helpful. I had originally written because a set of tabs had closed without the warning, and then when I subsequently checked in Options the checkbox I was used to was missing. However, since then the warning has returned, at least for now, so all is well. I do wish basic features like this would remain as a checkbox in Options, as changing things in about:config, even though I have done it a few times, still seems a bit daunting, and I don't think I'm alone in that feeling.

go into "options" then in "general" settings you can check box to warn on leave unchecked to not warn