It doesn't work, but you configure it in settings now. Open about:config and look for closeWindowWithLastTab
You don't need this extension anymore

It doesn't work. Closing the last tab closes the window.

Does NOTHING. FF still closes.

This doesn't really do anything. It doesn't even have an icon in the toolbar or "More tools...". Thanks to the user who mentioned about:config browser.tabs.closeWindowWithLastTab in their review, I don't even need an extension for this.

Work as described

Still closed the Firefox window after closing the last tab. Best solution I found: set "closeWindow" in about:config to false!

Piece of crap. Closing last tab STILL closes browser. WTF! Where is "Last Tab Close Button" extension?? It worked perfectly until sh*thole latest FF upgrade.

It does what the description says.

However, it is too quick in closing empty tabs. In the situation when several (or even really many) URLs are opened at once, FF cannot load them all immediately, so all but a few stay empty until their pages are loaded. DCLT closes these empty tabs too quick, so all but some of the URLs are not loaded.

Developer: please add a check for empty tabs: if there are pending pages/URLs to be loaded they should stay open.
Dirty workaround: add a (user-definable!) waiting time before empty tabs are closed.

There was an addon with exactly the same name (in Russian). So I decided that this is either a new version or a fake. That addon worked as follows - when the last tab was closed, the browser was not closed, but an empty tab was opened. Your add-on when closing the last tab does not allow the browser to not close.