Flaky UI tests on Firefox

[jtpio]

Description

The visual regression tests sometimes fail on Firefox.

This appears to be related to kernels not being shut down properly (or too slow), which means some menu items are still enabled:

Reproduce

Example run: https://github.com/jupyterlab/retrolab/actions/runs/1755661711

Expected behavior

This CI check should be less flaky.

[jtpio]

Closing for now, as these failures have been less frequent recently over on the https://github.com/jupyter/notebook/ repo.