How to delete the old Windows folders on old install -
https://superuser.com/questions/915173/delete-old-windows-program-files-from-second-drive
Steam - where are the game libraries? on ssd or on hdd? I would move them onto the hdd (mine already are), and then its really easy to reuse the game files on new install. All you do is install client, and go to settings and change the default library location to the old folder, and steam will find the games and remove the need to reinstall them again.
how to move games or add a library -
Steam games, yes. You can even have that other drive as the default location.
Steam games location
In the steam client:
Steam
Settings
Downloads
Steam Library Folders
Add library folder
To move an already installed game
Games library
Right click the game
Properties
Local Files
Move Install Folder
If your personal data on hdd is your documents library, you don't lose any save games then either
Firefox is another matter, you will need to reinstall it like most other apps on the new install.