To be clear, there's 3 background shown with 2 of them being the same color. 1) dark background from an unloaded browser frame 2) light background from firefox home page without theme set 3) dark background from firefox home page with theme set So the flash comes from firefox first loading an empty browser frame then actually loading the page content which by default says to show a light background until a later message arrives to make it show dark.
Bug 1603918 Comment 5 Edit History
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To be clear, there's 3 background shown with 2 of them being the same color. 1) dark background from an not-yet-loaded browser showing tabbrowser-tabpanels' background https://searchfox.org/mozilla-central/rev/c61720a7d0c094d772059f9d6a7844eb7619f107/browser/themes/shared/tabs.inc.css#31-45 2) light background from firefox home page without theme set 3) dark background from firefox home page with theme set So the flash comes from firefox getting ready to load a browser frame then actually loading the page content which by default says to show a light background until a later message arrives to make it show dark.