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Hello,

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Hello,

Type about:config<enter> in the address bar. If a warning screen comes up, press the Accept the risk and Continue button. At the top of the screen is a search bar. Type browser.tabs.warnOnClose and set it to true.

Hope it helps. Thank You.

@ Shashank Shekhar I set the browser.tabs.warnOnClose option from false to true, but it doesnt work. So I think I have to reinstall th browser to solf the problem

Reinstalling Firefox won't help. Firefox closes without warning by default, when you close the last tab. The check box to warn you when quitting the browser in the Firefox General panel under Startup is for when "Restore previous session" is enabled, to warn you when closing Firefox with multiple tabs open.

You should go to about:config and reset the browser.tabs.warnOnClose preference back to true (the default setting) so you are warned when closing Firefox with multiple tabs open if you later disable the "Restore previous session" Startup option.

You can try this solution, from https://support.mozilla.org/en-US/questions/1205166#answer-1077652

In about:config, find the preference browser.tabs.closeWindowWithLastTab and set it to false.

I tested in Firefox 77 on Windows 7 by setting this preference to false. When I tried to close the last tab, Firefox opened the New Tab page. To close Firefox, I had to use the X at the top of the window or go to the Firefox 3-bar menu and click Exit.