Request native setting/behavior to prevent flashing during WebExtension New Tab page load

[PaduaOne]

Description: When using a New Tab override extension (such as Tabliss) alongside a specific theme or mode in Firefox (v154+), there is a brief frame flash (black/white) during tab creation.

Requested Fix:

1. Expose a native about:config setting to set the initial background paint color for WebExtension-based new tab initialization.

2. Synchronize the initial window compositor color with the user's active theme or target background color to prevent flashbang transitions when opening tabs with Ctrl+T.

[Jon]

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