Don't really know how to think about the free client + proprietary server.
Proprietary is never good but on the other hand it's on the other guy's
server. (And if they're the copyright holders, then they can control the
software) Kinda perhaps like free web browser + proprietary web server?
Maybe somebody else has clearer thoughts on this. Possibly there is an answer
at [3].
Looking at the build instructions [2] that thing certainly can include FMOD
and KDU (which I believe is Kakadu, since it suggests to use the free
openjpeg instead). These are proprietary but it's possible to build it
without them, this probably means no audio. Any prebuilt binary will likely
contain them.
Apparently Second life has "sublicensed a proprietary physics library called
Havok for all viewers". [3] I don't understand if one can say no thanks to
this offer.
Apparently Second life contains some DRM too.
Looks like that opensimulator is c# and .net which is a pretty unfortunate
choice of language. Also, the mono framework is always lagging behind the
windoze version, by design... Of course, these don't make it non-free.
So, build it yourself and don't include FMOD and KDU and connect to Opensim
would be the freest combination.
[1] https://www.gnu.org/philosophy/who-does-that-server-really-serve.html
[2]
http://hg.phoenixviewer.com/phoenix-firestorm-release/file/ad5581d9b114/README_BUILD_FIRESTORM_LINUX.txt
[3]
https://metaverse4me.wordpress.com/2012/06/16/firestorm-for-opensim-and-more/
