Firefox has an option that lets you choose where to download things. I'd start by making a "!downloads" folder on the appropriate drive, then tell firefox to send everything it downloads to there.
Any software that installs pretty much has advanced options to let you choose where to install it.
Installing everything to C:\programfiles~ is just lazy in my opinion.
Also clean up your C drive, move things other than installed programs over to the new drive.
Such as downloaded (but not installed) software, apps, music, etc.