OK. Well we are forced by Mozilla to change extension format to something they call webextension which causes troubles for users using old version of Firefox. There is no way we can ensure backward compatibility so current public extension can’t be installed on such old Firefox with exactly same message you have mentioned. As download fails then once you have older extension in older Firefox it just won’t update and you will miss e.g. support for Credit cards we have just released in 17.7 version of avast for instance. You may also miss lot of security fixes Mozilla put in newer releases so I recommend update Firefox to latest version anyway.