<em:maxVersion>3.7a1pre</em:maxVersion>. And it works perfectly with my Firefox 4.01. The extension's page at addons.mozilla.org says that it's compatible with Firefox versions ranging from 2.0 to 6.*. If the extension itself does not state this compatibility, then it must come from somewhere else.
cmd_pasteNoFormatting. So all you have to do is to assign a new shortcut to the default "Paste" key binding and assign the Ctrl+V to this
cmd_pasteNoFormattingcommand. I've done it and it worked for me on Mac OS X and with TB 126.96.36.199.