"Dropbox users who run Firefox on Windows may want to check the plugins listing of the browser as the browser may have picked up the Dropbox Update plugin automatically after a recent update.Source: gHacks Tech News
"Mozilla, for unknown reasons, never fixed the automatic plugin installation issue in Firefox. While you can block the automatic installation of plugins manually in Firefox, there is no default indicator or permission system in place to protect users.
"Firefox checks various locations on the computer automatically for plugins and integrates them automatically.
"Programs too may install plugins in Firefox without informing users about that. This is one of the reasons why you have plugins like Google Update or the more recent Dropbox Update installed in the browser.
"Unless you check the plugins listing regularly, you may miss new plugin installations as there is no indication that new plugins have been added to Firefox. ..."
How to remove the Dropbox Update plugin from Firefox