I tried everything in the knowledge base. I have the Adobe Reader plug-in. It is set to Always activate. In Applications, I changed the pdf display option from Firefox Preview to Adobe Reader (for Firefox). All other Adobe options are set to Adobe Reader (for Firefox) The pdf files download instead of displaying in Firefox. I went to the next instruction to go to Troubleshooting and open my profile and delete the mime file indicated. It made no difference. The latest version of Adobe Reader no longer has the option "Display PDF in browser".