In case you’re using Google Drive frequently in conjunction with Firefox and always wondered about the random download locations (when always ask is enabled), I just found the reason for this…
Firefox has a boolean setting browser.download.lastDir.savePerSite (not defined by default) that defines whether to remember the download location per site. However, with Google Drive, you will get a random URL serving the file, turning the last known download directory into a useless feature unfortunately.
So, just go into about:config, and create the above mentioned boolean setting and use false as the value. Now you have a global last known download location.
Solution found here.