Custom CSS: Media Width label misaligned in Firefox

[MichaelArestad]

Looks like there was some old code that targeted Mozilla browsers (AKA Firefox) to fix some positioning specific on those browsers. It seems Firefox no longer needs that CSS and, in fact, the CSS actually makes it worse in Firefox so I removed it.

It's not clear why the code was added as it was ported into Jetpack three years ago and the original code is no longer easily available.

Changes proposed in this Pull Request:

• Removed CSS targeting Firefox to position a form label

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• Not a new feature. Nothing added.

Testing instructions:

• In Firefox, go to Jetpack's Writing settings and make sure Enhanced CSS is enabled.
• Go to Customizer.
• Select the Custom CSS menu item.
• Scroll down and look at the Media Width section to make sure the form label is aligned correctly.

Before

After

Proposed changelog entry for your changes:

• No changelog needed, but if you want to add one: "Fixed label position in for Enhanced CSS option in the Customizer for Firefox users."

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against 6a644ef

[crunnells]

LGTM