Amazon, will not stay dark, it is for a sec, but flashes to white. the top bar is black, where cart, etc is. I have Dark background and light text, darkreader addons (Mozilla) & picked custom (Black) on contrast for page in browser settings. Still white.
No idea, but since i also use automatic browser darkening, sometimes if the site has this option too, it has the opposite effect.
Because it is inversion, so a double inversion restores the original.
Just an idea, because it has happened to me. If the amazon settings have dark mode, try to toggle it off. Or add an exception in your plugin.
I suggest you speak to Amazon, or your Browser developer.
As @Ben indicates, use Dark Reader
Donb’t think it is settings. It started this recently. Maybe a week, my old motherboard was going ^ I thought that was it. Nope, still doing it. I even did the website contrast option in browser & made backgrounds black, still no.
I just had to re-do a bunch of settings that got reset - so it’s worth spending time checking it out and going through those settings.
Why are you so set in having Amazon dark? Are you staring at the page for ours on end?
He is visually impaired and needs high-contrast themes.
Just don’t enable more than one addon on one site since they will cancel each other. Which i suspect is already happening here.
Download Dark Mode for Amazon for Firefox. Experience Amazon in elegant dark mode. Perfect for night browsing and reducing eye strain while shopping.
Thanks. 
Definitely - Dark Reader should definitely not be enabled by default, you need to toggle it on when needed for specific sites - the same goes for anything else.
Any issues, then launch with a clean profile to test things out - if it’s clear then you’ve some homework to do in your profile - CtrlShiftA and turning off some of those extensions to see if you can clear it up.