Run Firefox CI tests in a separate job

When running CircleCI, the `test_unix` job runs the following jobs:

```yml
- run: npm run test -- --browsers Chrome,Firefox
- run: npm run test:integration:chrome
- run: npm run test:integration:firefox
```

Because the `test_unix` job is blocking all other tests, we have to wait for both Chrome and Firefox tests to complete before everything else can run. While it certainly makes sense to wait for one of them, waiting for tests of both browsers before we run everything else seems unnecessary. It would be better to split the `test_unix` job into a `test_chrome` and `test_firefox` job. We can run the Chrome job first, and run the firefox job in parallel with all other test jobs.

Find the CircleCI configuration that defines the test_unix job and inspect its Chrome, Firefox, and integration test commands. Split the workflow into test_chrome and test_firefox jobs, then verify that Chrome remains on the blocking path while Firefox runs in parallel with the other test jobs.

Written by the indexing model from the issue text.