you can toggle the preference described here: http://kb.mozillazine.org/Extensions.blocklist.enabled (the article is a bit outdated - the blocklist is now not only used for malicious extensions but also to disable vulnerable plugins & crashy graphic drivers).

• go to address bar and type "about:config?filter=extensions.blocklist.enabled" and hit enter
• Accept Warning
• toggle the value to false

I'm curious: which plugin is getting disabled? Usually Firefox won't disable a plugin if there's an update available.

Taking a glance at your list, you might want to update your Adobe PDF product. From within Adobe Reader XI or Adobe Acrobat XI, you can use

Help > Check for Updates

Firefox probably won't detect the update until after you restart it.