FF doesn't install plugins or help with plugin install and if you use that site to install it then you need to contact them for install options. FF can be installed Offline I've done that many times. Why don't you download that plugin your trying to install for offline install? Also if it doesn't met FF57+ requirements the plugin will not work or install as well.

Builtin plugins like OpenH264 are normally retrieved automatically when they are needed or possibly in idle time. These files are requested by contacting an update server for detail about what specific link to use.

There are media.gmp-gmpopenh264 (WebRTC) and media.gmp-widevinecdm (DRM) prefs for the default plugins.

This pref is evaluated to generate the URL to check for updates:

• media.gmp-manager.url

You can check the response in the Network Monitor of the Browser Toolbox.

• https://developer.mozilla.org/en/Tools/Browser_Toolbox

Toggle DRM off/on in Options/Preferences to force an update check.

See also this builtin JSON file that is used in case of issues with contacting the Mozilla update server:

• chrome://global/content/gmp-sources/openh264.json

WestEnd said

FF doesn't install plugins or help with plugin install. ... Why don't you download that plugin your trying to install for offline install? Also if it doesn't met FF57+ requirements the plugin will not work or install as well.

This is not about NPAPI Plugins.

The Fx 57+ requirement is about the Extensions and not NPAPI Plugins.

The poster was referring to the Cisco OpenH264 Plugin as mentioned say in https://blog.mozilla.org/blog/2013/10/30/video-interoperability-on-the-web-gets-a-boost-from-ciscos-h-264-codec/

Thank you everybody.

After comparing another PC setup with same OS and internet connectivity I did the following, in case it helps anybody:

• Copy the gmp-gmpopenh264 directory from the FF profile dir of the machine with internet connectivity to the FF profile dir of the offline machine.
• Compare the prefs.js of both PCs, in my case I had to add the following lines:

user_pref("media.gmp-gmpopenh264.abi", "x86-msvc-x86"); user_pref("media.gmp-gmpopenh264.lastUpdate", 1518864558); user_pref("media.gmp-gmpopenh264.version", "1.7.1");

I guess the lastUpdate doesn't matter that much. For some reason the version pointed by chrome://global/content/gmp-sources/openh264.json is 1.6 but the one downloaded is 1.7.1; perhaps I missed something or it's using some other URL. I guess that other URL comes from media.gmp-manager.url but I wasn't able to replace all the variables used there (e.g. https://aus5.mozilla.org/update/3/GMP/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml)

After restarting FF on the offline machine the plugin shows as installed now.