Ublock Origin and Vimium C. That's it.

I used Dark Reader until last week, when I discovered a native Firefox setting that does the job better: Settings > Language and appearance > Colors > Manage > set background to Black and override to Always.

No more white flashes, EVER (yes, I tried absolutely everything but on some sites there was nothing to be done, even with every possible CSS hack). And no more add-on speed penalty (to be fair it was small, and Dark Reader is still an amazing tool).

Now the web looks pretty ugly but it is fast and always dark. White flashes banished FOREVER.