Try the following three areas:
1. Desire for change. They are both dedicated to starting a change in their society; they are excited and they have a plan. However, Montag, as soon as he gained a more in-depth perspective of his society, and started reading, he immediately wanted to do something to enact change. Faber on the other hand, cowered in the background for years, not wanting to enact change for fear of what the goverment would do. He was silent for a long while, whereas Montag just found his voice, and is excited to use it.
2. Alienation. They are both men who feel ostracized from society, a bit alienated, and alone. They feel they don't have many friends, or people who understand them and their frustrations. Montag feels distanced from his wife, and Faber feels distanced from almost everyone. However, to contrast them in this area, Montag found someone who he felt close to, that sparked him and motivated him (Clarisse), whereas Faber just slowly distanced himself from everyone because he was afraid of getting harmed by society.
3. Jobs. They have both held professions that deal with books, so they are similar there. However, they differ in that Faber used books as a professor, taught from them, was educated and intelligent because of them. Montag is on the other spectrum of that; he burned books and never delved into them.
There are many other possibilities that you could use; for example, Montag takes an active role in rebelling (planting the book, killing Beatty) whereas Faber takes a more passive role, but, they both do rebel. You could focus on more shallow traits like their age and station in life. I hope that I have at least given you something to get started. I also provided a link below with tips for writing compare/contrast essays, and that should help too. Good luck!