I don't have the answer, but I can throw out a resource that might help find it.
Here are various options in firefox
It seems possible that browser.cache.disk.parent_directory, or something similar to it, is the option that you would need to set.
Although I can't be sure, as I have never set these options via command line myself. I have always done it via something like this, though:
FirefoxOptions ff = new FirefoxOptions();
FirefoxProfile profile = new FirefoxProfile();
profile.SetPreference("browser.download.dir", "%path%");
profile.SetPreference("browser.download.folderList", 2);
profile.SetPreference("browser.helperApps.neverAsk.saveToDisk", "text/csv,application/pdf,application/octet-stream,application/vnd.ms-excel,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
ff.Profile = profile;
Hope this helps more than no answer at all :)