I have a multi-part RAR download that was begun with Firefox but was just too slow. How do I use FDM to continue these downloads without either overwriting or completely starting these downloads from scratch (all have from 2 gigs to 5 gigs already downloaded.

That actually kinda depends on how you were downloading them.

Did you click a single download button and all the downloads were queued up in Firefox, or did you manually select files to all download in Firefox, or what?

It *should* be doable, but nothing is guaranteed. Knowing the methodology would definitely help establish if it is easily doable or not, though.

Make sure your have the FDM Plugin installed as well when you go to the site for the download link. Sometimes clicking on the download link starts the process and sometimes it takes you to a second screen that will start the process or FDM will popup asking to download and if so click "yes" and it should start. Sometimes the site will not support Resume so if it breaks the link you will have to start over. So be sure to look at what process it uses to download either you have "Resume or it says no Resume" options-when on those sites.