Firefox includes a feature that lets you use multiple identities (multiple accounts) on one site, with each account in a distinct, colour-coded "container". What this means for NationStates is that you can be logged in to multiple nations in different tabs all at once, without having to switch between them. This allows to greatly speed up card farming, as switching between nations can take a considerable amount of time. Of course, the more farming puppets you have, the greater the time savings.
In essence, containers can be thought of as if you're using a different browser (or private browsing) for each nation, but all within a single browser instance.
This feature stems from Firefox' desire to be a privacy-focused browser. So far, it is exclusive to Firefox, and won't work in Chromium-based browsers (Chrome, Edge, Opera...).
One of the many extensions leveraging the power of Firefox' containers is Containerise, which lets you automagically open a tab with a different, logged-in nation (without having to log out from all of your other ones) by simply clicking a link.
TL;DR: Containers are a Firefox exclusive feature that enables you to be logged in to multiple nations without having to switch and relogin.
You can install Firefox through your package manager or download it here.
The default Multi-Account Containers extension is cumbersome to use with lots of puppets: each separate container must be opened manually through clicking in a menu.
Containerise is an extension that lets you dynamically change the nation you're logged into (the container) based on the content of the address bar. You can get it here.
This is the most confusing step of all. Containerise uses a special syntax, Regex, to choose when to switch to a different "container" (nation).
There's two main ways in which you can configure the extension to switch to a nation: nation=puppet or container=puppet.
Instant switching when visiting the puppet's nation page (ex. nationstates.net/nation=my_secret_puppet switches to "My Secret Puppet")
Does not work on N-Day/Z-Day and possibly other events (it will switch you to the puppet, preventing you from shielding/healing it)
container=puppet has to be manually added to the address to switch to a puppet (ex. nationstates.net/container=my_secret_puppet to switch to "My Secret Puppet")
Does work on N-Day/Z-Day (will not switch you to the puppet unless container= is in the address)
If in doubt, nation=puppet is usually easiest for beginners and will work out of the box.
You will have to generate a rule for each puppet, like this:
@^.*\.nationstates\.net/(.*/)?nation=puppet_number_1(/.*)?$ , Puppet Number 1
@^.*\.nationstates\.net/(.*/)?nation=puppet_number_2(/.*)?$ , Puppet Number 2
@^.*\.nationstates\.net/(.*/)?container=puppet_number_1(/.*)?$ , Puppet Number 1
@^.*\.nationstates\.net/(.*/)?container=puppet_number_2(/.*)?$ , Puppet Number 2
The part before the comma is the puppet's name, all lowercase, spaces replaced with underscores. The part after the comma is any name that uniquely identifies your puppet (purely cosmetic, but it has to be unique — I recommend just using the puppet's name). The easiest way to generate these rules is with a spreadsheet.
Having done this, click on the Containerise extension's button (four colourful squares).
TL;DR: Read the damn step, it's important.
It's a good idea to generate clickable links to each of your nations, so that it automagically opens a new container-tab. If you don't want to use the script from the previous step, use a spreadsheet. You can also add additional links that redirect you to any NS page, like your puppet's issues or it's deck page.
The links should be in this format:
Issues link: https://www.nationstates.net/container=my_puppet_1/page=dilemmas
Issues link: https://www.nationstates.net/nation=my_puppet_1/page=dilemmas
TL;DR: Too lazy to read an optional step? Your loss.
Now that you've configured the extension comes the mundane task of having to log in to each of your puppets. The upside is you'll only have to do this once!
Open a new container-tab according to the rules format you chose:
Click the Login button
Enter your credentials and click the Stay logged in/Remember me checkbox.
(If you don't you'll have to login next time — that defeats the purpose of containers)
Click the Log in button
TL;DR: You'll figure this out sooner or later. Or you could just read the 81 words.
There's a lot of scripts created by the community that you can use to speed up farming even more.
I've created a set of 3 simple ones to make issue answering easier (they'll have to be all installed).
The rest of the repository contains other useful cards-related scripts, I encourage you to take a look at them too.
If you're new to cards, I recommend reading this excellent guide by DGES/TNP's Cards Guild.
You can also join the cards discord for more tips, tricks & tools.