I updated to firefox 5 today and got the same "This page requires a plug-in that can only run in 32-bit mode" as I did when I updated to firefox 4. I clicked the "restart in 32 bit" button and firefox restarted, brought me to netflix where I was when I restarted, and I got a pop up saying silverlight has an update. I updated silverlight and then closed and re-opened firefox and all was well. I have a feeling that if you don't get the pop up about updating silverlight you can just go and do it yourself from the microsoft silverlight website and it should work.

oh, I'm using firefox 5.0 on OSX 10.6.7 but I'd assume that if your having this same problem on a windows machine updating silverlight would work as well.

tl;dr restart in 32 bit, update silverlight, restart in 64 bit, watch movies.

hope this helps!

Sorry, I forgot to take this down I fixed it a while back.

Restarting in 64-bit mode does NOT work (at least with Firefox 5). Whenever I do this, I just get the same message streaming Neflix: "Thanks for installing..."; this page requires a 32-bit plugin; restart Firefox in 32-bit mode. That always happens when I try to stream Netflix in Firefox UNLESS I select "run in 32-bit" in the "Get info" dialog for the Firfox app. Then, of course, Firefox will ONLY run in 32-bit mode. This limitation is partly a Firefox problem; Safari WILL run a 32-bit plugin (eg., Silverlight) even when the application is running in 64-bit mode. The Neflix problem is MS Silverlight. Not even the latest Silverlight 5 beta is a 64-bit app (That feature is supposed to be available in the final release, "...by the end of 2011"). However, I don't know why Silverlight limits Firefox to 32-bit, since this doesn't seem to be a problem for Safari.