[Swipe tabs] Darker square becomes visible when swiping right on the first tab in the tab list

Steps to reproduce:

1. Open Firefox.
2. Make sure you have more tabs opened
3. On the first tab in the list, perform a right-swipe gesture.
4. Observe the UI during the swipe.

Expected result: Only the background color should be visible during the swipe interaction

Actual result: A darker square is visible during the right-swipe gesture.

Device & Build Information:

• Device: iPhone 14+ (iOS 16.0.3)
• Build version: v143.0 (60740)

:information_source: Reference Person

@FilippoZazzeroni

┆Issue is synchronized with this Jira Bug