I remember the white flash before turning a page to dark theme, that was long time ago. It eventually went away by itself after a while, don't know if it was FF's fault or the dark theme's fault.
My current FF (on Ubuntu) is v116.0, and I doesn't flash. Regarding dark theme, I'm using a FF extension called 'Dark Reader'. However, for EEVblog only, the 'Dark Reader' is set to not change the page, and I'm using another extension instead, called 'Stylus'. Stylus will let you apply your own format to a given webpage. There is one Stylus format ready to download for EEVblog dark themed online, or use the one from here:
https://www.eevblog.com/forum/chat/forum-dark-theme/There is also a dark theme for EEVblog forum that can be selected from the forum settings, no browser extensions needed.
Another FireFox-like browser is LibreWolf. LibreWolf is compatible with FireFox plugins/extensions, and has tighter privacy and security. Maybe LibreWolf won't flash.
For YouTube, I'm using a FF extension called 'Enhancer for YouTube', can change many settings, including a YT theme, theater mode, default playback speed etc. Can play full screen 1080HD/60Hz at 2x playback speed, with the internal video card of an i4970k proc. YT at 4k/60Hz plays smooth in full screen, but only at x1 speed. At 2x speed it may become choppy sometimes, so I've set the 'Enhancer for YouTube' to only use 1080 even when in full screen.