Job offer Accepted but Interview Offered at better job

Looking for some advice. I've been unemployed for a year now. Finally have a job offer for a charity. I love the cause (they are human rights based), the job is flexible and remote. I accepted it and signed the contract. The only flaw is it's a fixed term contract. Due to start next month.

Problem is, I've now been offered an interview for a different charity. It is better paid, flexible remote, a senior position and permanent. But the cause isn't exciting. Do I consider it? Worried about burning bridges after signing a contract and not sure what is best to do...

Thanks!