It depends on the MOD and if it has any conflicts with changes made in 3.0.9, though it is quite likely that a sizable fraction of MODs developed for 3.0.8 will also work without problems in 3.0.9. The best way to be sure would be to ask the MOD authors directly to see if they have confirmed that it will work with 3.0.9.
Alternatively, if you do not wish to wait for a response from each individual MOD author, you could install a test installation of phpBB 3.0.9 and test each MOD there. That way, you do not risk jeopardizing your live forum and you can find out which of your MODs also work on 3.0.9. I would refrain from trying to install these MODs on your live 3.0.9 board until you are sure of their compatibility.