MacOS – How to join call using “Skype for Business Web App” on macOS Catalina

Using my Mac I'm trying to join a conference call hosted using Skype for Business Server. But every time I try, it seems to send me in loops with the same prompts to install the plugin again and again. Here's a screenshot of the screen I start with.

1. Join with Web App

If I enter my name and click "Join the meeting" my browser downloads SkypeForBusinessPlugin.pkg (md5sum = a4958d5aa5c7dbe17774bcd6e4492106). I can run this and choose to "Install for me only". After installation I see a new plug-in in ~/Library/Internet\ Plug-Ins called SkypeForBusinessPlugin-15.8.bundle, so the installation is apparently successful. The bundle ID is com.microsoft.skypeforbusiness.plugin. I don't know which browser this is for, but I assume it should work in Safari. When I go back to the same page after installing the plugin, I am either prompted to install the plugin again, or I get this error message:

Plug-in installation is taking longer than usual. We recommend you join the meeting now.

If I try to join at this point I end up in a "Virtual Lobby" but I'm never able to advance from that point, presumably because the plugin is not working. Could the plugin be incompatible with 64-bit macOS Catalina?

Join using Lync for Mac

I tried the second option of using Lync for Mac, but when I try to run Lync I get this error:

“Microsoft Lync” needs to be updated.
The developer of this app needs to update it to work with this version of macOS. Contact the developer for more information.
https://support.apple.com/en-ca/HT208436

So I guess I can't use Lync for Mac on macOS Catalina.

Known workarounds

I installed Skype for Business on my iPad and had no trouble connecting to the call that way. But I want to use my Mac for this.

Additional details

I'm using macOS Catalina 10.15.3 (19D76). My normal browser is Firefox 74 (64-bit).