The browser.tabs.warnOnClose preference should not reset itself to false after you set it to true, unless Firefox is Reset/Refreshed. But keep in mind that if the number of tabs open in the window is less than 24 (the number you can restore immediately from the Recent Closed Tabs list), then you won't get the warning even with that preference set to true.

Meanwhile, I'm thinking there must be a more efficient way to get a count of open tabs. Maybe this add-on: https://addons.mozilla.org/firefox/addon/tab-count-icon/

Note that you can move the button to the Tabs bar, if you like, after you "Pin to Toolbar" from the Extensions button drop-down. See:

Step 1: Manage your extensions using the extensions button in the toolbar

Step 2: Customize Firefox controls, buttons and toolbars

Do you mean the close X to close a window? Firefox doesn't ask for confirmation if the number of tabs is less than what session restore can handle (reopen).

You can paste this code in the command-line in the Browser Console to see the total number of tabs.

gBrowser.tabContainer._allTabs.length

For pinned tabs:

gBrowser.tabContainer._lastNumPinned

To enable the command-line in the console.

• https://firefox-source-docs.mozilla.org/devtools-user/browser_console/
• select "Enable browser chrome and add-on debugging toolboxes" in the Web Developer Tools Settings (F1 in a Web Developer Tool)
• https://firefox-source-docs.mozilla.org/devtools-user/settings/