There is no built-in feature for this now and I can't remember one before. Do you mean if it's the last tab and Firefox would close?

For many pages, you can recover by using Undo Close Tab -- either right-click a different tab and click that on the context menu, or press Ctrl+Shift+t to restore the last closed tab. But some pages will reload and discard your form input, so that's a nightmare. There are some add-ons to help with that which you could look at, such as the following (I have not tested them myself in recent years):

• https://addons.mozilla.org/firefox/addon/form-history-control/
• https://addons.mozilla.org/firefox/addon/textarea-cache/

Or, another idea:

You may have seen from time to time that a site asks if you mean to leave. (For example, if you start a comment on Reddit and decide to close the page.) This is an in-page script, but in theory you could inject it into other pages where you want a warning about closing or leaving accidentally. I wouldn't want to click that box all day every time I navigate a tab, so it would make the most sense to only do it on pages where you care about losing data, and not just everywhere you type into a form (like every search).

I might experiment with that.

Finally, you can try the Advanced Search feature here: https://support.mozilla.org/search/advanced

I did an advanced search and found this answer from cor-el:

set this pref to false on the about:config page to prevent closing window when you close the last tab.

 browser.tabs.closeWindowWithLastTab

For help using the about:config page, see the Configuration Editor for Firefox article.

P.S. (from another answer I found) With that setting, when you close the last tab in a window, Firefox should open a new tab to keep the window open. Then you can undo the close tab.

jscher2000, thank you so much. I did not know about the "Undo Close Tab" option. It completely solves my problem. Thanks again and again and again.