After refreshing firefox or gtk-common-themes while firefox is running, it fails to load icons from the theme

I'm testing a snapd snap built from https://github.com/snapcore/snapd/pull/10676 and I can confirm that firefox doesn't crash anymore when it's running and any of the following snaps is being refreshed: snapd, core20, gnome-3-38-2004, gtk-common-themes, firefox.

That's a significant improvement over the current situation (see bug #1945697 and https://bugzilla.mozilla.org/show_bug.cgi?id=1756083).

However, when refreshing either firefox or gtk-common-themes, the application then fails to load icons from the theme (and I'm seeing the corresponding GTK warnings) when e.g. opening the Downloads window (Ctrl+Shift+Y):

(firefox-beta:19252): Gtk-WARNING **: 15:30:44.644: Could not find the icon 'window-minimize-symbolic-ltr'. The 'hicolor' theme
was not found either, perhaps you need to install it.
You can get a copy from:
 http://icon-theme.freedesktop.org/releases

(firefox-beta:19252): Gtk-WARNING **: 15:30:47.507: Error loading theme icon 'go-previous' for stock:

(firefox-beta:19252): Gtk-WARNING **: 15:30:47.508: Error loading theme icon 'go-next' for stock: