I think normally if the download arrow has information on the panel, it would remain visible. To avoid the "auto-hide" behavior, you can turn that off.

Call up Customize: Customize Firefox controls, buttons and toolbars

Left-click the Downloads button and uncheck the box for auto-hide

Of course, it's possible it won't have any information for you after that, but at least it should always be there so you can check it without having to call up the full downloads list.

Auto hide is enabled but trying another download the download indicator remains until I either close Firefox or view the Download.

This was not the case for me. the download was not viewed on my high sierra Mac.

Note that in current Firefox releases the auto-hide setting is also available in normal mode via the right-click context menu (Auto-Hide in Toolbar).

This issue has occurred again while using version 67. I was downloading Virtualbox and the download indicator disappeared with no evidence of the download. This is on a Mac running high sierra.