However, I'm worried about some other big extensions that have really been useful not being able to be ported to WebExtensions, like DownThem'All, the developer is pretty pissed at Mozilla[1] and offers some insights about the design decisions made by Mozilla that could have a big impact on the ability of some AddOns to tap into the APIs they need to do anything useful.
[1]:http://www.downthemall.net/re-downthemall-and-webextensions-...
[1]: https://mail.mozilla.org/pipermail/dev-addons/2016-December/...
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.