⌘+Delete does not work in Firefox
It doesn't work for me either. You can use control-U to delete a complete line, but it also deletes the part on the right side of the caret.

Or if you use KeyRemap4MacBook, add something like this to private.xml:

&lt;item&gt;
  &lt;name&gt;firefoxcmddelete&lt;/name&gt;
  &lt;identifier&gt;firefoxcmddelete&lt;/identifier&gt;
  &lt;only&gt;FIREFOX&lt;/only&gt;
  &lt;autogen&gt;__KeyToKey__ KeyCode::DELETE, VK_COMMAND | ModifierFlag::NONE, KeyCode::CURSOR_LEFT, ModifierFlag::SHIFT_L | ModifierFlag::COMMAND_L, KeyCode::DELETE&lt;/autogen&gt;
&lt;/item&gt;