Plugins may be installed by copying (or symlinking) the plugin related modules into
~/.mozilla/plugins. If you need to put them in a custom location, you can
export MOZ_PLUGIN_PATH=<path1>:<path2>... and run Pale Moon after this.
You can put these export statements in
/etc/profile, should you need them to have effect for all users.
It is preferable to install Flash from the official repositories if they have one. Sometimes a "non-free" branch must be enabled in the package management tool to get closed source software through the repositories.
To install it from the GZIP file available at Adobe's Flash Player distribution page, copy the contents of
/usr directory in the archive to the system's
/usr and the
Pale Moon reads the standard Firefox directories for global extensions, thus extensions may be globally installed extensions may be extracted into
/usr/share/mozilla/extensions/ or, for a specific user, in