18th February 2017, 22:13
I think I've somehow solved the issue, though it might be a crazy solution. I found other 2 parameters:
What I did is, reset the first one to empty and set the second to false. Then restarted firefox and with the default (ugly) firefox directory picker, it used the last dir for downloads correctly. Then I set the picker to true again and after a restart it also works fine with the Plasma picker. Strange, maybe something got reset, but seems that it works. Hopefully, it might help someone in the future...
Code:
browser.open.lastDir was set to my Desktop dir
ui.allow_platform_file_picker set to true by defaultWhat I did is, reset the first one to empty and set the second to false. Then restarted firefox and with the default (ugly) firefox directory picker, it used the last dir for downloads correctly. Then I set the picker to true again and after a restart it also works fine with the Plasma picker. Strange, maybe something got reset, but seems that it works. Hopefully, it might help someone in the future...