insists on opening Safari as default browser for WebAssembly

I have Firefox configured as my default browser, and "default" as my browser in QtCreator's WebAssembly configuration. When I try to run a WebAssembly build, the following happens:

if Safari is not running: Safari is launched, shows "Compiling..." status, then hangs there. Firefox opens a new tab, loads the build, and runs it.

if Safari is already running: Safari opens a new tab, shows "Compiling..." status, then hangs there. Nothing further happens, Firefox is not invoked.

So now I have to command-Q Safari every time I launch a build... this is with Qt 6.4 (and the corresponding emscripten). With Qt 5.15 the same setup worked fine.