Hover preview in about:config keeps resetting to true after restarting

I want to turn off the hover preview feature. First I went in settings, but that disabled only the thumbnail (browser.tabs.hoverPreview.showThumbnails in about:config is set to false and saved). Then I went in about:config and manually set browser.tabs.hoverPreview.enabled to false as well, that disables the unwanted feature completely.

But every time I restart the browser it's turned back to true (just hover.Preview.enabled, not hover.Preview.showThumbnails). Can I save my settings? It's annoying that I can't opt out.