Are the brown lines, with or without odor, that appear on underwear days after wearing it considered an impurity that prevents prayer, given that they are sometimes present while in the mosque?

Brownish threads are not considered impure based on doubt about their nature; rather, there must be certainty that they are impure. If you become certain that they are impure, but you do not know when they appeared, or you prayed unaware of their presence, then your prayer is valid and you are not required to repeat it. However, if you are certain of their impurity and aware of their presence, then they must be purified, and prayer is not valid with them.