Where are Firefox addons stored?

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Question

Where are Firefox addons stored?

Answer

In Firefox: Open Application menu > Help > More troubleshooting information,
or go to about:support

See the location of the Profile Directory:
/home/USER/.mozilla/firefox/john.default

The extensions are stored within zip files in the extensions/ sub-directory, like:
~/.mozilla/firefox/john.default/extensions/

With the zip.vim plugin, you can easily view the content of the extension:

~/.mozilla/firefox/john.default/extensions/ $ vim plasma-browser-integration@kde.org.xpi


" zip.vim version v32
" Browsing zipfile /home/overshoot/.mozilla/firefox/john.default/extensions/plasma-browser-integration@kde.org.xpi
" Select a file with cursor and press ENTER


action_popup.html
content-script.js
action_popup.js
extension-historyrunner.js
action_popup.css
constants.js
extension-mpris.js
extension-tabsrunner.js
content-utils.js
extension.js
extension-kdeconnect.js
extension-downloads.js
extension-purpose.js
extension-utils.js
i18n.js
manifest.json
options.html
options.js
options.css
permission-request.js
permission_request.css
permission_request.html
utils.js
...

The extensions and their corresponding ID are listed in:

about:support<code>.

You can also run the following code within the browser's console and study its output:
<code>
var {AddonManager} = Components.utils.import("resource://gre/modules/AddonManager.jsm", null);
AddonManager.getAllAddons().then(addons => {
    for (let addon of addons.filter(addon => addon.type == "extension"))
        console.log(addon.name, addon.getResourceURI().spec);
});

Some add-ons are stored system-wide in:

/opt/firefox/browser/features/