I upgraded to the latest version of Firefox 11.0. I figured out how to download. Click save and it downloads, but still doesn't open in browser.
I'm sorry for probably being unprecise. However, I'm not quite sure if "Display PDF in Browser" is supported for Firefox on Mac at all.
Was that working before you updated to 10.1.3?
The latest Acrobat/Reader update 10.1.3 provies a plugin for viewing pdf in Firefox version 10.0 and later.
Try running Firefox in 32bit mode (via Applications -> Cmd+Click on Firefox icon -> Get info -> Check "Open in 32bit mode")