Note that the kapersky extensions that are installed in your browser do nothing to increase your security, but are only a way for the anti-virus makers to make more money through search revenue and data mining. The actual anti-virus that actually protects your computer runs in the background and runs no matter what browser you are using (or if you are even using a browser at all). Kapersky needs to sign their extension so it will work in future versions of Firefox, but until then there is no harm in leaving it disabled, in fact it is probably better that way.