Firefox, is a web browser from the Mozilla Foundation
Thunderbird is an email client for reading and sending of emails, and more, also from the Mozilla Foundation,
these both came from the Netscape Communications Corporation camp of the mid 1990’s
The Browser, is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. This could be one of the following :- Edge, Internet Explorer, Chrome, Opera, Safari, to name some of the common ones.
The start page is just that.. where the brower ‘starts’ . Its normal to have a ‘home’ button to go back to the start. common start pages are google.co.uk, yahoo.co.uk, or bing.com but they normally can be configured to a page of your choice. vendors and manufactures like to steer the user their page.