Fix Firefox CI issue

[hsubox76]

Github's latest image for ubuntu is ubuntu-22.04 which seems to come with a version of Firefox that causes the karma launcher to hang and eventually give up when trying to launch Firefox. Other users have reported the Firefox bin isn't there at all, which is not exactly the same issue, but could be related. Locking it to ubuntu-20.04 until we can figure out why (hopefully it isn't that current and future versions of Firefox don't work with karma-firefox-launcher).

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7a03687

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (fde5adf) and merge commit (618a06b).

Test Logs

• Base (fde5adf): https://github.com/firebase/firebase-js-sdk/actions/runs/3527399174
• Merge (618a06b): https://github.com/firebase/firebase-js-sdk/actions/runs/3568436846

1. https://storage.googleapis.com/firebase-sdk-metric-reports/j6ZdM2h6dR.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (fde5adf) and merge commit (a1dba14).

Test Logs

• Base (fde5adf): https://github.com/firebase/firebase-js-sdk/actions/runs/3527399174
• Merge (a1dba14): https://github.com/firebase/firebase-js-sdk/actions/runs/3569034844

1. https://storage.googleapis.com/firebase-sdk-metric-reports/CoyOSLAOVM.html

[dwyfrequency]

LGTM

[dconeybe]

Thansk @hsubox76!