I've used it for a long time, on desktop and mobile Firefox. Sometimes if, for instance, I find a person with a red name, like on reddit maybe, I might check through their post history and see some of the other things they've said, and I've often struggled to find anything related to their views on trans people, scrolling and scrolling through hundreds of comments and posts and never finding a thing. And other times it's immediately apparent. I imagine for a lot of just regular people on the internet, getting their name red can be as simple as sharing an ignorant opinion once, twice, a handful of times, but not appearing to actively espouse anti trans bigotry.
Since there is no, like, way to add a note to an account that is being marked, or to save a web address linking to the post or event or thing that made somebody flag them, there's never any way to be sure why an account is red or not unless you just happen to stumble upon the post that caused the flag, or if they frequently express their bigotry such that any of it would be easy to find.
Plus some people using the plugin might just not understand the rules, and mark people who just are trans as green, instead of people who do pro trans activism or strongly voice support for us, like the rules say. So, for red names, maybe someone has never shared an opinion on trans people but they just give off a "vibe" that makes a person mark them red. Or, one could even be a troll or anti trans person who downloaded the plugin to start flagging wrong on purpose just to screw with us, I don't think there's any protection against that scenario. So you just can't know for sure.
For my purposes, I'm always wary of red names/links as a rule, and generally more trusting of green names, but if the topic at hand isn't about trans people at all, I may overlook the colors entirely, though I tend to let that information guide my interactions with the marked. I have found, though, that whole websites, subreddits or notable internet personalities, like their twitters or YouTube accounts or something, which were marked one color or the other, tend to be more reliable than just a random individual's social media account. More eyes, more chance a person would see something sus and flag it again.
I would agree with the reviews that said they use it as a suggestion, it cannot be definitive with how it works, but that doesn't really take away from it I don't think, it's a good warning system for not wasting your time somewhere or with someone that doesn't respect trans people, as a trans person. Just sometimes you have to ask "why would this account be marked red/green?" and to consider that when doing your mental calculus about interacting with a certain person/site/etc or not.
Edit: fixed some words
Reminder that the phrase "may you live in interesting times" is a curse against the recipient. Lots of boring nothing happening for multiple generations is the best that anyone can hope for. How I yearn for boring...