If you are concert that the eBooks will exploit your eBook reader and infect your device you can use a website view them like this https://epubreader.net/. They will have to escape the browser sandbox to infect you’r device witch is extremely difficult and generally don’t happen if you keep Firefox updated and you are not “valuable” target.

I would disable file thumbnails in your file manger.

Ecosia is not that great for privacy “Your IP address and search terms are automatically shared with our search partners, Microsoft Bing or Google, when you search.” https://www.ecosia.org/privacy

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Thing worth adding they no longer use cloudflare like it says on their blog they have switched to ddos-guard.