Is working as an accountant in a company that relies on bank loans and Murabaha, takes out one loan to pay off another, and deals with fictitious companies to obtain loans, considered unlawful (haram) and thus must be abandoned?

It is not permissible for you to work in the aforementioned company as long as you are an accountant there, because your work will not be free from tracking interest-based loans and passing forged documents for fictitious companies, which are Islamically forbidden matters. Also, because the company relies on borrowing and banking usury, and in that there is assistance in sin. Allah Almighty says: (And do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty).