Multi-Account Containers allows you to keep the websites that you visit separate from each other.

Often, people use them to allow them to access multiple accounts on the same website. For example, having a Personal container and a Work container would allow you to sign into both email accounts at the same time.

Another benefit to containers is to protect your privacy. For example, if you keep your social media in your Personal container, their advertising tracking code can't track what you are visiting in your other containers like Work or Shopping or Banking.

You can find out a lot more about this feature on the Multi-Account Containers page.

Hope this answers your question.

Thanks for the explanation.

I wanted to know what the message about this addon and other specific addons is about.

What makes these addons unique to containers?

Why do they require container tabs (what does "require" mean) ? Container tabs seems to be enabled by default (I assume there must be a way to disable it, even tho the checkbox is greyed out).

Since Multiple Tab Handler still works, what prompts that message?

This is about the permissions that an extension requires.

• https://addons.mozilla.org/firefox/addon/multi-account-containers/
• permissions: [<all_urls>, activeTab, cookies, contextMenus, contextualIdentities, history, idle, management, storage, tabs, webRequestBlocking, webRequest]

In this case it includes contextualIdentities and when an extension needs this permission then container are enabled automatically.

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contextualIdentities

So basically, upon installation, an addon that requires certain permissions will automatically enable the Containers feature.

If, for whatever reason you don't want Containers enabled, you can't use the addon(s).

Also, only one of potentially multiple addons is displayed. It turns out that if you decide to disable that addon from within the options, and if you have other addons that fit the same criteria, they will appear, one by one, until you have disabled them all, at which time containers becomes de-selected.

IMO, It's all a little awkward and confusing. I've got close to 50 addons, yet only 4 met the conditions for the contextualIdentity requirement. I'm left to wonder what makes the Multiple Tab Handler permissions so unique as to require contextualIdentities. They look pretty basic to me.

On the surface to a layman like me it all seems kinda unnecessary.

The only thing that I can think of is that the MTH extension comes with support for containers and thus requires this specific permission. You can contact the author/creator of the extension if you have a question about the permissions it requires.

Thanks for all the help. I've got a better handle on it now.