@denno,
Macromedia DW MX activation & registration servers were taken offline many years ago. At the time, Adobe issued a replacement serial number that does NOT require activation. Assuming you have still have your installation disks and a compatible OS that can support legacy software, scroll down to part 3 below to obtain the MX 2004 serial number for Win or Mac.
https://helpx.adobe.com/x-productkb/policy-pricing/macromedia-legacy-activation-error.html
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That said, an outdated code editor may be hurting more than helping you. Validate the code in your sites and fix all reported errors to minimize potential problems on modern devices.
Test your sites' performance on these metrics to ensure you're doing all you should be doing.
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Last time I looked, Blue Griffon and others have no file management/FTP features. At a minimum, you'll need a S/FTP or FTPS client like FileZilla, CuteFTP, CyberDuck, Transmit, WinSCP or etc...
https://www.pcwdld.com/ftp-and-sftp-clients-for-windows
Dynamic sites will require more robust tools to handle modern PHP, JavaScript, Python, etc...
-- Adobe Brackets (open source) - http://brackets.io/
-- Adobe Dreamweaver CC - https://www.adobe.com/products/dreamweaver.html
-- Wappler - https://wappler.io/
-- Visual Studio Code (open source) - https://code.visualstudio.com/
Good luck!