So, watching Lovefilm on a Mac sucks right now.
OSX Snow Leopard.
Latest Silverlight plugin straight from Microsoft.
Firefox 15 or Safari.
Once I worked out how to unblock the SL plugin in FF it worked, but the next day I was getting a plain black rectangle instead of the Watch Now buttons or play controls. If I right-(control-)clicked the box I get a "Silverlight Preferences" option but when I choose it, that app fails to launch. Same when I try the "Microsoft Silverlight" option straight from Applications.
If I remove the plugin by deleting the .plugin file from Application Support then reinstall it, SL works long enough to watch a film.
Where are the useful log files such that I could diagnose the conflict or permissions failures that might be causing this?