From MozillaZine Knowledge Base

Plugin path

Mozilla applications on Linux consult several directories looking for plugins. The exact list depends on how the application binary was compiled and packages, but can include directories such as

$HOME/.mozilla/plugins
program_directory/plugins
/usr/lib/mozilla/plugins
/usr/lib/xulrunner/plugins

where $HOME is the user's home directory and program_directory is where the Mozilla application binary resides.

Some Linux distributions search additional directories to handle the Adobe Flash player plugin or to support 32-bit plugins in a 64-bit environment.

null plugin location

Firefox installs a default plugin named "libnullplugin.so". Finding it will reveal the location of a plugin directory.

find / -name libnullplugin.so -print

Fedora Core 3/4

Where Firefox has been installed with the FC3/FC4 distribution, /usr/lib/firefox-<version>/plugins where <version> is a numeric reference to the version e.g. /usr/lib/firefox-0.10.1/plugins