I've never seen an htm.part file but, I've encountered quite a few mp4.part files. In the latter case, the download is in two parts - a) the video and b) the audio. In my experience, I believe it has something to do with Windows Shell Experience Host, simply Refreshing the download destination folder completes compiling the two. Besides, if they were malicious, Norton should detect it.

Firefox immediately starts downloading the file in the background and during the download uses the .part file and updates this file when new chunks arrive (the file is closed in between chunks and reopened for the next chunk). When the download has finished then the .part file is moved to the actual destination folder and the .part file extension is removed. If you keep the .part file then your AV software might be causing problems and keep an handle on the .part file.