In these days the net is populated by blog posts about the .NET Framework Assistant plugin for Firefox, its disabling and the fact Firefox does not warn user when a plugin installs itself without explicit permission.
Well, this is a old problem at least for me, indeed I discovered it some time ago when my Firefox crashed (apparently) without reason, discovering after a couple of days that Microsoft Office 2003 plugin fought with Foxit Reader and Google update plugins.
The problem was that I never installed Office plugin!
Disabling Office plugin Firefox stopped to crash.
After that experience I decided to write a simple (very simple) extension that at every Firefox startup checks if there are new plugins installed.
Waiting Mozilla implements a better solution than mine you can install PluginChecker.
If you expect to find PluginChecker innovative or the “I-can-live-without-it” extension you are on the wrong place, if you expect a not intrusive and simple solution to unattended plugins installation than you can take a look at it.
You can download PluginChecker from SourceForge.