I'm pretty interested if it is possible some way. I tried to change the default download path in prefs.js, in the registry (getadmx.com/HKLM/Software/Policies/Mozilla/Firefox), even if I change the windows default downloads folder location in the explorer, non of them work if the prompt for download location is set. It always starts in the username/Downloads folder.

I think that you probably will have to lock the browser.download.lastDir pref using an autoconfig.cfg file to prevent Firefox from changing the pref to the last used path.

// first line is a comment line
lockPref("browser.download.lastDir", "<file-path>");

Note that on Windows you need to escape '\' as '\\' as you can see if you inspect the current value in prefs.js (about:config shows the actual path).

• https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig

Does not work for me. On the about:config page I can see my new location under browser.download.lastDir, and it is locked, but download window still pops up in the default download folder.

Same if I set any download parameters as group policy in the registry. It is set, locked, but download window pops up in the default download folder.

Did you trey to lock the default download folder as well (browser.download.dir) ?

Yes, I did. As soon as I set the browser.download.panel.shown to true, it's just not gonna use the folder I set.