Inconsistent Window Size on Initial Browser Launch (RFP issue)

[Tho3ke]

Steps to Reproduce:

1. Open the browser when it is not running.
2. Take note of the window size. ( https://arkenfox.github.io/TZP/tzp.html )
3. Open a new window while the browser is already running.
4. Compare the size of the new window with the initial window.

Expected Behavior:

The size of the first window opened should be consistent with the size of subsequent windows.

Actual Behavior:

The first window opened is slightly larger than new windows opened when the browser is already running.

Additional Information:

This is mainly issue for users with privacy.resistFingerprinting enabled which is apparently disabled by default recently.