For awhile my left breast is larger then my other. i feel no lump just feels firmer then the other. no history of breast cancer. likely possibilities?

Hard to tell: It is not uncommon for women to have one breast larger than the other but if this is a new thing and if as you say it feels "firmer", i would advise you visit your pcp or gyn as soon as you can for a good exam.

Need evaluation: It could a benign ' gynacomastia ' or could be a tumor , you need to see your doctor for further investigation., breast cancer is rare in men , but 1% of them are in men.

Breast asymmetry: This can be related to many causes most are benign like gynecomastia and related to medications or use of marijuana. It is more concerning that it is unilateral and thus you should see a surgeon to evaluate and get imaging as indicated.