Where is Firefox's plugins directory?

Please read the update at the end of this answer regarding changes in Firefox 52.

Acording to the Firefox developer documentation plugins live here:

• Directory pointed to by MOZ_PLUGIN_PATH environment variable
• ~/.mozilla/plugins
• /usr/lib/mozilla/plugins (on non-Ubuntu 64-bit systems, /usr/lib64/mozilla/plugins might be used instead)
• Plug-ins within toolkit bundles
• [Profile directory]/plugins, where [Profile directory] is a user profile directory

Update for Firefox >=52

As of Firefox 52 support for NPAPI plugins has been dropped. All plugins living in one of the above directories are ignored, with the exception of Adobe Flash.

If you installed Flash with the flashplugin-installer package, it's loaded through a symlink in /usr/lib/mozilla/plugins that ultimately links to /usr/lib/flashplugin-installer/libflashplayer.so.

I am on KDE 32bit 14.04 version. Firefox is already installed.

user Richard got negative reputation by telling us the correct path:

/usr/lib/firefox-addons/plugins

If you are proper linux user, it's no problem to move libflashplayer.so file to this directory as root user!

Youtube works after dat!

Users should place plugins in /usr/lib/firefox-addons/plugins as root.