Bug 1659631 - readme: update the wording to firefox, DONTBUILD.r=sylvestre

Differential Revision: https://phabricator.services.mozilla.com/D87226

README.txt

-An explanation of the Mozilla Source Code Directory Structure and links to
+An explanation of the Firefox Source Code Directory Structure and links to
 project pages with documentation can be found at:
 
     https://firefox-source-docs.mozilla.org/contributing/directory_structure.html
 
-For information on how to build Mozilla from the source code and create the patch see:
+For information on how to build Firefox from the source code and create the patch see:
 
     https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html
 
-If you have a question about developing Mozilla, and can't find the solution
+If you have a question about developing Firefox, and can't find the solution
 on https://firefox-source-docs.mozilla.org/, you can try asking your question on Matrix at chat.mozilla.org in `Introduction` (https://chat.mozilla.org/#/room/#introduction:mozilla.org) channel.
 
 
@@ -17,5 +17,5 @@ Nightly development builds can be downloaded from:
             - or -
     https://www.mozilla.org/firefox/channel/desktop/#nightly
 
-Keep in mind that nightly builds, which are used by Mozilla developers for
+Keep in mind that nightly builds, which are used by Firefox developers for
 testing, may be buggy.