This extension worked fine for me before. But since one year my different conainters don't remember my 2 gmail accounts, so i'm forced to relog each time. Reinstall of firefox doens't solve the matter.
If someone has any other advice or other extention to recommend, i'm all ears.

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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

Using FF 72.0 & Multi-Account Containers 6.1.1 it lasted for about 15 mins then broke. I cleared all my history including the cache and after that container assignment in the URL tab disappeared forever. It previously appeared using a mouse right click on the tab. Tried bouncing everything and reinstalling it...still dead. I can manage the containers but putting a tab in one is gone. Finally got it working by completely reseting FF and reinstalling it along with my other add-ons.

no possibility to sync with other instances of firefox. Its useless this way. The whole syncing makes for me only sense because I can use the browser at home and at work, but i dont want to mix the contents. Because Firefox does not support multiple profiles, or does not have a user friendly support for profiles, the multi-account-containers would be a good workaround, but if you cannot sync containers, this addon its useless.

In times where privacy both matters and is at risk, it's great to keep your logged in sites in something that looks like a different browser (the containers). Besides it also allows to have different accounts on a given website and be logged to both of them at the same time, just in different containers. This is the best thing Mozilla could have made for browsing. A fantastic enhancement on Private Mode (which is a bit like a container too, but it does not preserver your logins, history, etc., whereas containers do.)

Makes using this functionality easy. Needs Firefox Sync! Need to add ability to set a preference for each bookmark, which container to open in when clicked!

Extension indispensable pour qui possède plusieurs comptes sur le web. Avec lui on peut créer autant de containers que l'on veut, les nommer chacun par l'adresse mail souhaitée ... et ouvrir 2 , 10 comptes gmail simultanément (ou autre fournisseur de mail), mail ou site quelconque où l'on accède par au moins 2 adresses mails différentes.
Problème, les containers créés ne sont pas sauvegardés, donc on peut les perdre.
A peine signalé et c'est corrigé, désormais c'est sauvegardé avec sync et donc disponibles sur tous les pc, Merci :)

This add-on is basically the only reason I use Firefox, and it has so many issues that I frequently contemplate leaving it.

Syncing across multiple devices (Windows, Mac, Linux) just doesn't work. It doesn't. On Windows I have 4 duplicate "Home" containers that reappear every time I sync. On Linux I get over 10,000 "Personal" containers. On Mac, who knows. I delete the extra containers (or try, on Linux), and next time I sync a device, they're back! It's maddening!

Now on Linux I've uninstalled and reinstalled a dozen times, and it won't add my custom groups. Where did they go? Why is this so hard?

good

this extension has been very helpful. i see its true value everytime i use firefox.

An app that is not only great, but also necessary!