You don't know before the update if an extension is supported or not. Firefox 56 doesn't support all of the new features of web extensions so some add-ons (and versions) are only available for Firefox 57.

Too bad Mozilla changed the Add-On website a few days ago. You don't see any indication of supported Firefox version anymore. You only see the extensions that work in your current browser, not the extensions for the next version. (Unless you switch to the old layout in an link hidden in the footer.)

Some extensions will be delayed because the deadline (2 years?) wasn't long enough. Some extensions need additional features and may only be available with Firefox 58 or later. Some never.

And you need to invest some time to research this. One starting point is https://arewewebextensionsyet.com/