Can't use temporary custom addon

I’ve written a webextension addon that I use in Tor. I’ve been loading it by going to about:addons, going to the Debug Addons page, and then loading it as a temporary addon (since I don’t want to break my privacy by getting it signed).

This used to work in an older version of Tor Browser - the toolbar icon for my addon that I registered via browser_action in the manifest would appear, and I’d be able to use it.

However on Tor Browser 91.4.0, loading the addon doesn’t do anything. While the addon appears in the list of temporary addons, the toolbar icon doesn’t appear, neither in the toolbar nor in the toolbar customization menu. I’ve added a few debug console.logs in my background script and they don’t appear in the inspector.

The same thing happens with the “beastify” addon in Mozilla’s webextension-examples repository - the addon “loads” but the icon does not appear.

I can confirm that my addon loads OK in Firefox 95.0.2, so I don’t think it’s an issue with my code.

Has there been a recent-ish change in Tor that would block addons from working properly, or is this a bug?