If you had Adobe Reader installed but didn't have the Firefox plugin for that installed, Firefox may have asked you if you wanted to install such a plugin. Firefox also now offers a built-in PDF reader, though I'm not sure if that gets installed by default vs. being asked if you wish to install it as an optional add-on whenever you download a PDF, if you don't already have a different reader plugin installed.
Since un/reinstalling Firefox didn't help, I suspect the problem is with your Firefox Profile, which is where all your bookmarks, settings, add-ons, etc. get kept between upgrades or un/reinstalling. See the link I provided above to create a new, fresh, blank Profile. I'd suggest uninstalling Firefox, then restart your computer, then reinstall Firefox, then before you start up Firefox again normally, do that new-profile routine.