Mozilla is an open source group of not-for-profit group of software engineers who compete with for-profit companies such as Microsoft and Google.
Mozilla's free browser is Firefox and their free Email Client is Thunderbird
Firefox is considered better than MS's Internet Explorer and as good as Google's Chrome. Definitely better than Netscape.
A 404 error happens when the browser cannot find a web page. Often because it is under repair or just plain erased. The problem is at the other end, not a fault with your system.
https://www.fordbarn.com/404.html