There's actually a way to prevent some, or all, plugins from "installing" but keep addons in Firefox. The problem is that on Windows, Firefox scans for plugins on some locations. On each of these locations can reside one or more plugins.
You can control how Firefox treats each of these locations but you cannot control how it treats individual plugins if the location lists more than one. Doing this involves multiple methods. Not all are nice.
Before getting into it,
it's worth to mention that there is also click to play, that will ask you to click before activating any plugin individually, if a webpage asks for any. It can be found in about:config as plugins.click_to_play. You can set it to true.
Now, first the nicer parts:
You shall go to the about:config page and filter for plugin.scan. Here, plid means a registry key location. The rest is for plugins that are handled individually.
plugin.scan.SunJRE", "1.3"
plugin.scan.Acrobat", "5.0"
plugin.scan.Quicktime", "5.0"
plugin.scan.WindowsMediaPlayer", "7.0"
plugin.scan.plid.all", true
The "1.3", etc. is the minimum version number that Firefox accepts.
The recommendation is that for plugins you want to disable, set this value to 19.0. I've set it to 99.0 and it works in Firefox 18.
You can check the actual location of these plugins by the means described on Mozilla's KB. The idea is that you go to the about:plugins page to see active plugin's location. But you need to set plugin.expose_full_path to true first.
To disable all registry-based plugin location scan, set plugin.scan.plid.all to false. The actual registry key is described on another page the KB. I listed the keys later.
If you ever need to reset these about:config settings, right click on them and choose reset.
Now for the uglier part:
Firefox will look for plugins in <Firefox installation dir>/plugins. This was where plugins like Acrobat, QuickTime and Office installed themselves in my computer.
The setting that was controlling this directory was removed. That's why I choose to modify the access rights of this directory. Set the rights for all users to read-only. Then no new program will be able to write to it.
I guess you could do the same with the registry key, too. The actual registry keys are:
HKEY_CURRENT_USER\Software\MozillaPlugins\plugin-id
HKEY_LOCAL_MACHINE\Software\MozillaPlugins\plugin-id
And on 64-bit Windows:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\MozillaPlugins\plugin-id
(To set permission on a registry key, you right-click it and go for "Permissions...". You shall see it, but if don't try starting regedit with the tool psexec -s -i regedit.exe or using regedt32.exe if I recall it well.)
Just for the record, I've removed all access from the system user for the plugins dir, and Firefox didn't crash. Any new installation may, though.
For archiving purposes, I include the full url's as text here:
plugin scanning: http://kb.mozillazine.org/Plugin_scanning .
registry key: https://developer.mozilla.org/en-US/docs/Adding_Extensions_using_the_Windows_Registry#Plugins .
location: http://kb.mozillazine.org/Issues_related_to_plugins#Plugin_location .
I've found these articles by browsing a category page listing all kind of interesting articles: http://kb.mozillazine.org/Category:Plugins
