I've had this add-on for several years, and I considered it useful until it came to light that in the past couple of years it's been marking certain people as anti-trans for... supporting nonbinary, intersex, and transmasculine people? I don't know what happened here, but this means that a tool that was once meant to help a vulnerable set of people has now turned into some sort of guessing game as to how reliable it is.
I rated this two stars instead of one because I've only encountered this issue on Tumblr whereas everywhere else (especially Twitter) it's been accurate. The only reason I still have it is to try and correct some of the falsely flagged ones I see.

EDIT: Seeing all the empty accounts here trying to claim this extension still works as intended and that the various transmasculine, nonbinary, intersex, and neopronoun users speaking up about this are wrong has made me bump this down to one star. What a shame for what used to be such a useful add-on.

This used to be really useful for weeding out terfs and their alt accounts, but now it's marked a ton of trans people and intersex people as terfs because a white trans woman got mad at them. Unfortunately, I can no longer recommend this extention

Shinigami Eyes was great in the beginning. But transphoobic activists have basically destroyed its fundamental usability. Accounts from intersex people are almost all marked red, trans men too, random trans friendly blogs too. On the other side, so many unapoligetic transphobic accounts are marked green, which makes this addon less than useless. It actively damages the community as long as there are people hoping for actual indicators.

This add on has been wrongfully targetting trans men and intersex people. Not recommended.