I'm using Nightly (Firefox 24 compiled by Dave Yeo). I used it for a while, then my plugins disappeared. Even though all the files are still there in the plugin directory, Firefox no longer uses them.
I created a new profile, and the plugins are back. So I used that profile for a while (moving all my bookmarks and passwords to the new profile), but in a few days, my plugins are gone again. I made yet another profile, and they are back.
Any idea what is going on? Is there a way to restore plugin capability to a profile?