I have had this problem for about a month, and finally figured out that Kaspersky Parental Control was to blame. No matter what domains I add to trusted sites, and no matter how "little" control I choose, I just can't get Google drive to load. Neither can I get standard Gmail icons (arrows, the Google logo, icons like "trash" and "inbox" buttons) to show up.
I think the problem began after a Kaspersky update, but I can't be sure. Gmail and Gdrive are not the only unrecognized sites. For example, in trying to resolve the problem, I went to update my plugins one by one, and I couldn't even update Quicktime because the site was not trusted, and I didn't have an option to override it or add it as a trusted site! I tried updating Quicktime three or four times before I gave up.
Unfortunately, this means that Kaspersky Parental Control is useless for me on Firefox.
However, IE will still load Google Drive, even if Kaspersky Parental Control is enabled. Bummer that we now have to use IE instead of Firefox. :(