Search notes:

Browser add-ons

Firefox

At least in the world of Mozilla/Firefox, add-ons is an umbrella term for different types of code that modifies Firefox. Add-ons include

Extensions (which can be thought of as an application). Extensions were formerly referred to as Web Extensions.
Themes
Plug-ins
Dictionaries
etc.

See also https://addons.mozilla.org/

In 2018 Firefox stopped supporting the old XUL extensions API in favor of Extensions («WebExtensions»).

Firefox (and Thunderbird) add-ons come in *.xpi files (which is actually a zip file). They're installed under ~/.mozilla/firefox/abcd5678.default/extensions

Google search link fix: to copy links from Google's search result pane.

See also the extensions folder and the addons.json file in the profile folder.

User agent switcher

https://addons.mozilla.org/de/firefox/addon/user-agent-switcher: Change the user agent

Apparently, this user agent switcher is not compatible with Firefox Quantum.

https://addons.mozilla.org/en-US/firefox/addon/uaswitcher seems to be compatible with Firefox Quantum, but does not allow to specify a user defined agent!

Finally: https://addons.mozilla.org/en-US/firefox/addon/user-agent-string-switcher (by RayLo) is both compatible with Firefox Quantum and allows to create my personal user agent.

Links

http://bernaerts.dyndns.org/linux/74-ubuntu/271-ubuntu-firefox-thunderbird-addon-commandline