Re-add Firefox to browser list for tests

[kptdobe]

Follow up issue of #247.
Tests were randomly failing because Firefox could not properly start. Removing it from the list was the only solution.
Once microsoft/playwright#35183 and modernweb-dev/web#2907 are fixed, we can re-add it.

[kptdobe]

For the record, the issue seems to be fixed on playwright canary build. We just need to wait for the next playwright release (1.52.0?)