docs(css): Remove Firefox select element default

[adamdbradley]

Request Type: docs

How to reproduce: View the docs from Firefox. The version picker's select elements arrow is still showing.

Component(s):

Impact: small

Complexity: small

This issue is related to:

Detailed Description:

Other Comments:

In firefox the version picker's dropdown from the default select element is still showing. This is a hack to force FF to hide the ugly default.

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#6878)

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Thanks again for your help!

[mary-poppins]

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