Error -- newest version of Waterfox -- Non-restartless extensions no longer supported

[Kyleri]

I just installed the newest version of Waterfox (2020.04, 64 bit) & upon trying to reinstall Echofon, it claims that the file is corrupted. I'm using the version made by YoruNoHikage back in January.

When I try debugging the addon, though, it gives this error:

There was an error during the temporary add-on installation.
Non-restartless extensions no longer supported

So I figure that something, somewhere, needs to be changed.

As this is very much beyond my clue, I appeal to y'all in the hopes that someone who can actually code can make it Do the Thing.

Many thanks!