Disable firefox e2e tests in CI

Created by: lguychard

Even though Firefox e2e tests pass in CI, the step hangs ("Jest did not exit one second after the test run has completed"): https://buildkite.com/sourcegraph/sourcegraph/builds/44680#4d8fc6ca-b8fc-4d22-954f-d08d8af3cfd5

Disabling this to unblock releasing the browser extension while we look into why this is hanging.