This part of the docs specifies what permissions cannot be optional permissions, and it doesn’t include unlimitedStorage:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Request_the_right_permissions#Request_permissions_at_runtime
But this part of the docs specifies permissions can be optional permissions, and unlimitedStorage isn’t there, either:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_permissions#API_permissions
So, does Firefox allow it or not? (If you know it does not, I’d appreciate you filing a bug.)