Make sure you are not accidentally still logged in to the account with the MS Windows session. Uncheck the checkmark that says to keep you logged in when you log into Facebook. It won't work when you use Ubuntu, because the account is being used. I have come across this many times when setting up Yahoo accounts, Facebook, etc.
I have never adjusted the MTU, it has worked just fine with the default settings...and I have set up hundreds of dual boot, and Ubuntu Only systems over the years, so that is something new and strange that Facebook would not work in Ubuntu straight out of the box. :-)