Which download progress bar is the problem: one of the ones on the little panel that drops from the toolbar button, or in the Downloads list in the Library window, or something displayed by an add-on?

Download Manager Tweak is generating that error for other users, according to recent reviews:

https://addons.mozilla.org/firefox/addon/download-manager-tweak/reviews/

I suspect it's not practical to fix that just for Firefox 56 since Firefox 57 will make it incompatible anyway.

So is there a way to monitor download progress within Firefox other than with that add-on?

If you click the arrow button on the toolbar, each download in progress should be listed. Actually, that might only show the three that started most recently; to see everything, you can use the full Downloads List (Ctrl+j, launches in a separate window).

In FF 57 the green arrow showing download progress has been moved next to the address bar (black thin, kind-of-underligned down arrow. Addons for showing download progress are no longer compatible with FF 57. The only othe way to check downloads is to click Tools->Downloads (Ctrl+J) to open up a Downloads page.