Why is Fire widely seen as breaking Effy's character?
I know this is an unpopular opinion, but I think Jess Brittain wrote Effy well in Fire. At first glance, it might seem strange that Effy in Fire actually cares, but considering her aversion to being dependent and becoming an object of pity, it's no wonder that after the events of Season 4, she wanted to show herself and the world that she is "fixed." She shows her need for stimulation through how much she feels stifled by her job as an assistant and how naturally she fits into the world of high finance, as well as through her risk-taking and cunning—like when she goes to a meeting with an investor without authorization while still an assistant. Her manipulativeness (obviously). Her sharp mind—like when she finds the error in the report and how quickly she picks up the trading lesson from Dominic. And her defiance—like when she doesn't let Victoria walk all over her. The fact that she has to be "the smartest person in the room"—just as with the mushroom incident where she wants to secure her position as the center of the group during the camping trip, in London she similarly can't just be a regular, beginner trader slowly learning her craft; hence the initial insider trading move. Her agency, like at the end when she takes down Jake and makes sure Dominic doesn't face any consequences. How caring she can be toward those she loves, visible in the scenes with Naomi in the hospital. Her ability to listen, evident in the scene with Naomi in bed. Her loyalty, for instance, when she fulfills Naomi's request by not telling Emily about her illness. The fact that she has a bullshit detector, like when she tells Naomi not to hide the truth from Emily. The fact that she doesn't confide her feelings even when going through a tragic situation—like during her last conversation with Naomi, when asked what she is going to do, she says nothing, even though her romantic and professional life lies in ruins and she herself is facing prison. The fact that she feels guilt when someone she cares about suffers because of her—in this case, Dominic, just like before with Freddie, which is visible in the final scene of Episode 5 of Season 3 (I know this is controversial, but in my opinion, she isn't mocking Freddie there; instead, shock and then shame are painted on her face, noticeable when she briefly looks down as Cook appears behind her; of course, pride wouldn't let her look away for longer, but nevertheless, there is no arrogant "Effy smirk" there), or during the conversation scene with JJ in Episode 7 of Season 3 (by the way, it is incorrect to blame Effy entirely for the breakdown of Cook and Freddie's friendship, where Cook can act maliciously or even cruelly toward Freddie; similarly, Dominic is an adult, experienced man in the industry, and Effy never pretended to love him—she doesn't let him kiss her on the terrace, and when Dominic tells her: "I'm hypothetically in love with you. As you know," and she answers: "It's just a crush, you twat."). The fact that she has a certain kind of humility—both when Emily takes her anger out on her for not telling her about Naomi's illness and when JJ accuses her of causing the breakup of the "three musketeers."
Many criticize the storyline of her relationship with Jake, but notice how quickly she becomes emotionally attached to Freddie, and in both cases, it happens during difficult moments for her: here it is Naomi's diagnosis, and there it is her parents' marriage falling apart. There is also her unwavering loyalty toward Pandora, even though Pandora betrays her trust by sleeping with Cook during the slumber party. And her assistance to Jake stems from the same protective instinct she showed when helping Tony get Michelle back. Even details like using clubbing as a form of escapism, shown in the sleepless night scene. The fact that she still doesn't shy away from drugs, shown in the company party scene. Her sharp comebacks, shown after Emily's comment during the dinner scene of her visit from New York. Even the fact that she has a nice, straightforward friend in Jane, who feels reminiscent of Pandora, also fits her character.