.lit is a proprietary format that is used by Microsoft's Reader software. A quick check of the features page for fbreader at http://www.fbreader.org/about.php revealed no listed support for .lit. If you need to convert your (totally legal, I'm sure) .lit files you can use any of a number of utilities to do so. ConvertLIT at http://www.convertlit.com looks to be a decent one. I'm not sure whether you will be able to strip out the DRM though if the file wasn't originally licensed to your copy of Reader.

This took me about 30 seconds of searching to find out. How about you spend your 30 seconds next time?