Hello

Here are some questions above all ...

- On which site did you install the plugin?

- Did an error code appear?

- Is this an error during the download?

Here are some solutions

- Reinstall the plugin

- Directly install the plugin on the official Firefox add-on for modules (https://addons.mozilla.org/android)

- Repair Firefox

If you still have trouble with the installation of plugins, do not hesitate, ask us!

Cordially

Julien

edit: fixed A.M.O url as it was to desktop Firefox section.

Is this with Android Firefox or actually the desktop Firefox for Windows, Mac OSX, or Linux.

If this is actually a Extension then extensions made for desktop Firefox will not just work on Android Firefox also unless it was made to do so.

The Android Firefox did not really use NPAPI Plugins much and support of the rather old Flash Player 11.1.x branch was dropped as of Android Firefox 56.0 and later Releases.

For desktop Firefox the only NPAPI Plugin allowed to run in Firefox Releases since 53.0 is the Flash Player Plugin from Adobe. https://support.mozilla.org/en-US/kb/npapi-plugins