*.htm is a web page, same as *.html.
You can set FP to be the default HTML editor in the options somewhere (by
adding the file extension *.htm" to the list of files that FP will open for
editing by default. e.g. you can set it to open ASP, ASPX, PHP, HTML, HTM,
JS, and others if you wanted to. The option is under the options menu (not
sure where specifically, but I think it's "Editor Options" or something).
Hope this helps.