Hi Guys,
I'm not sure if it is already logged as a bug (with mozilla) but there seems to many posts on different forums about the fact that accepting email invitations in TB does not work when hitting the "accept" button.
What worked for me is to simply drag the .ics attachment onto the calendar pane on the right-hand side. The appointment then pops up and you can then save it. (It does not always seem to work the first time though...)
I use TBP 2.0.0.14 and lightning extension 0.8.
I hope this helps.
Regards
Paul