I"m using Firefox on Ubuntu 12.04 and up until now I've been able to view PDF files within the browser. Now it tells me that I'm missing the plugin. I don't know which plugin it is. I have the add-on PDF Viewer 0.7.1 that allowed me to view PDF files within Firefox. I would just like to view PDF files within the Firefox browser again instead of downloading the file everytime since I have to use my school's Blackboard system to access the files. Please help.