Firefox, for whatever reason, doesn't support the OS X-standard ⌘Delete shortcut for deleting the current line of text.
How do I get it back?
10.8 with FF22, but this behavior has been around for a while.
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:
<item> <name>firefoxcmddelete</name> <identifier>firefoxcmddelete</identifier> <only>FIREFOX</only> <autogen>__KeyToKey__ KeyCode::DELETE, VK_COMMAND | ModifierFlag::NONE, KeyCode::CURSOR_LEFT, ModifierFlag::SHIFT_L | ModifierFlag::COMMAND_L, KeyCode::DELETE</autogen> </item>