Firemacs, Firefox extension which gives emacs keybindings.
C-S-t in Firefox will bring it back. Don't know if the content of the text field remains though.
It does. It's not that hard to test.
Seems the keybindings skip direct to the semantics in your brain, without passing through verbose names; C-w would "kill" the text.
Still, your lapse is pretty good. Sometimes I can't even name the keys -- but my fingers know :)
For uninitiated, C-w is "yank text", i.e. cut it, and Firefox it is "close current tab without warning".