Upload messages from Thunderbird to GMail

I'd like to have all my old messages from Thunderbird exported to
GMail. Can anyone think of some clean way to do this? I mean sent mail
being in sent folder and inbox having proper names as senders.

Btw. when I select more messages in Thunderbird, it disables plain
forward and only options is forward as attachment (as one message), is
there a way to get round this?

And if it would actually work I am not afraid of complicated methods,
because I'm doing this just once anyway.