The boiling point of halogens increases down the family.
Boiling Point
Boiling point has multiple factors, but for the purposes of this question, the main factor is IMFs. IMFs are intermolecular forces. IMFs are the attractions that occur between the atoms or molecules of a sample. The stronger the IMFs, the higher the boiling point. This is because strong IMFs hold the molecules together, so it takes more energy to break the attractive forces.
Polarizability
Pure halogens are always non-polar. This means that there are no dipoles or other forms of permanent electron shifts. Since all halogens are nonpolar, there is only one type of IMF: London Dispersion Forces. LDFs are the attractive forces caused by the temporary and random shifts of electron clouds. All molecules experience LDFs. However, LDFs have varying strengths. The larger the molecule, the stronger the LDF. This is due to polarizability. Polarizability is the ease with which electron clouds can shift. Bigger molecules have larger electron clouds, hence the higher polarizability and stronger LDFs.
Since halogens further down the table are larger, both in atomic radius and electron cloud, they have higher polarizability. This leads to stronger LDFs and IMFs. As stated in the first paragraph, strong IMFs cause higher boiling points. This is why larger molecules like Bromine have higher boiling points than small molecules like Fluorine.
