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THIS WONT WORK (THE EASY WAY):
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Misleading quote: "Avoid leaving social-network footprints all over the web (for example, you could use a Container tab for signing in to a social network, and use a different tab for visiting online news sites, keeping your social identity separate from tracking scripts on news sites)."

You'll need to add an option to change its behavior. When opening a site in a container (e.g. Bing-search) - clicking a link - the new (unassigned) site will stay in the same container thus enabling tracking. Pretty useless this way. Although useful for using different profiles, it's not secure/private.

Options to add (user-decided behavior):
- Open new unassigned sites in the default container, a temporary container or current container (you can reassign later)
- Switch/assign (via toolbar button) the current site to different container (assign/move/copy to a new container for site)
- Changing to a new container will open a new tab or use the current tab (a new tab to avoid losing *cursing* history)
- Option to open web sites in their own containers without the user assigning a container (auto-created for TLD's) *
- Possibility to select a default container for a site assigned to multiple containers or use last container used
- Possibility to sort container labels
- Add an overview of which site/data is contained in which container
- Try to also use the omni-box for extra functions (assigned container switching) since the container-label is already there

* When reassigned by user the auto-created container is renamed or merged with an existing container

EDIT 1: Rating lowered from 2 to 1 star until problems are solved. Totally wrong implementation (the omni-box becoming useless and is imprisoned by this add-on). I get the idea but it's wrong. Visiting Facebook, then entering a different URL, will keep you in the Facebook container. Track-track-track. Just too much concentration and handling is needed to keep things real secure. For now this extension is just a kind of identity switcher within the same Firefox user-profile. Why not then just create an add-on to switch to another user-profile with a toolbar drop down-menu? At least that's clear and transparent. This surely isn't! For this to work properly you've to create a container for every URL/cluster of sites you visit. We need Firefox Web Clusters/Containers.

EDIT 2: Found out that sites are currently not even linked to containers (only cache/cookie data) which was not my assumption. It starts your selected container and stores data in that container separating data from other containers. The problem here is that selecting a default container for a specific site results in not being able to switch accounts for that specific site. A contradicting (dis)function.

Abstract/Résumé:
- You can switch profiles by selecting containers in the toolbar-button (entering the URL again)
- Ones assigned to a specific default container, you can't switch profiles
- You'll need to be careful entering websites you're not willing to have in the container since this enables tracking *
- To protect you from tracking you'll need (an) extension(s) that isolates Facebook, etc. from all other containers **

* You'll notice you're logged off from web sites (don't log in but start a new container). This extension is (not) for dummies.
** There's a Facebook container that doesn't work i.c.w. other forks (Google, Twitter, etc.). Also you can't switch profiles.

EDIT 3: For a workable solution install these extra add-ons

- Switch Container (https://addons.mozilla.org/en-US/firefox/addon/switch-container/)

- Temporary Containers (https://addons.mozilla.org/en-US/firefox/addon/temporary-containers/)

Moz://FifthAxiom