Hello,
great plugin. I've compiled it yesterday for FIrefox 1.5 on Ubuntu 5.10 and it works great so far. However, I've used GCC 3.4 (no luck compiling current MPlayer with GCC 4 - although I didn't try too hard) and I've had a problem with the extern declarations of strlcat and strlcpy in plugin-setup.cpp. The functions are in a .c file, so their names are compiled according to C rules, however these external references, being in a .cpp file, are created according to C++ rules. A simple adding of the "C" flag (extern "C" ... ) solved this problem. Maybe the source distribution could be changed this way too.
FYI: so far no crash on the mentioned configuration (Ubuntu 5.10, Firefox 1.5, MPlayer 1.0pre7try2-3.4.5). I've used  gecko-sdk-i686-pc-linux-gnu.tar.gz from 20th November 2005 (being closest available to the 20051111 that Firefox 1.5's About box declares).
Best regards,
   Ondrej