I'm not sure, but I don't have many apps installed and it didn't happen before the 7.1 update.

Something similar has been reported in the past as an MDNSD issue.

https://bugzilla.mozilla.org/show_bug.cgi?id=1215012

https://forums.oneplus.net/threads/firefox-high-battery-usage-fix.339787/

It's weird that it happens even after restarting the phone and not running the app. My phone would be extremely hot though, so it seems like the CPU is being ran in the background.

I also tried turning the "battery optimization" for FireFox (in Android battery settings) to On and let it sit overnight but that didn't help the issue. I had to turn "battery optimization" Off because video playback in FireFox was very choppy with it turned on.

I don't think my Nexus 6 would have lasted very long if I hadn't uninstalled FireFox, it seemed like it was running the CPU at 100% until the phone died.

FireFox was my preferred browser on Android since it allowed uBlock to be installed, I'm still trying to find a reliable alternative and making due with Chrome for now.