And as for a DownThemAll fallback whose feature subset may be enough for you, consider Download Star: https://addons.mozilla.org/en-US/firefox/addon/download-star...
Does it allow you to manage your downloads though - like supporting multi-source downloads, pausing and re-starting, exporting/ importing lists, prioritise, re-order, limit speed, integrating with the save dialog and menus ...
DownThemAll is the one extension that might force me to abandon firefox or to stick with an ESR release and not upgrade.
As for "not upgrade": do stay on ESR for as long as possible and hope more fully-formed alternatives for your favorite addons eventually arrive. But really, don't disable upgrades altogether and risk being pwned due to running an unpatched, insecure browser.