Add custom download location for Firefox mobile

[SpikeFiend]

 Allow the user to select a custom location for downloads of images and files from the FireFox mobile browser.

FireFox mobile will also likely require "All Files" access for FireFox mobile to be able to deal with the scoped storage in Android 14+.

[Jon]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.