Changing FIREFOX_VERSION from devedition-latest to nightly-latest

[jamesmortensen]

…fix #1670

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Description

Nightly is the correct FIREFOX_VERSION as devedition-latest is the beta version with extra developer tools baked into it.

Motivation and Context

firefox:dev version should be on the bleeding edge.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.