Hi,
Most of the people who answer questions here are other users volunteering their time (like me), not Mozilla employees or developers.
If you want to advocate for the ability to customize download location for Firefox for iOS, we highly encourage you to vote or chime in on this idea on Mozilla Connect, our official feedback platform.
That said, this is a known issue. However, no work has been initiated on it yet. We recommend sharing your feedback on Mozilla Connect, our official feedback platform.