I do not think the battery reports for Firefox on Android are correct, though it definitely uses a lot more battery than other browsers. Some of the statistics from the battery use were ridiculous and made no sense.
Mine looked similar on my Pixel. I could not use Firefox at all and it would somehow use 40% of the battery, according to the monitor. That's after restricting the background use and force stopping the app as well.
I stopped using it recently and my battery life has noticeably improved. I'm upset about this as it's otherwise a great browser.
