"I think about firemen all day."
Translation:Jeg tenker på brannmenn hele dagen.
No, 'om' but here we have to decide between 'å tenke om' (= to have an opinion about) and 'å tenke på' (=to think about).
For example: Jeg tenker dårlig om ham. - I do not like him.(this wording is unusual though)
Jeg tenker på deg. - I am thinking about you.