Firefox should not automatically reuse authenticated containers for external links by default

[Kan_McNonexist]

Product: Firefox Desktop
Version: 160.0 (Release)
OS: Windows 11 Enterprise LTSC 2024 (Build 26100)
Extensions: Firefox Multi-Account Containers, Temporary Containers
Related bugs: 1692124, 1874599

Summary

Firefox automatically opens links from external applications inside existing containers, even when the user has never explicitly assigned the website to that container.

I consider this a privacy-unfriendly default that undermines the purpose of container isolation.

Steps to reproduce

1. Create a container using Multi-Account Containers.

2. Open chatgpt.com inside that container and sign in.

3. Do not configure "Always Open This Site In" for the domain.

4. From Windows PowerShell, execute:

   Start-Process 'https://chatgpt.com/'

5. Observe which container Firefox selects.

Actual behavior

Firefox automatically opens the external link inside my existing authenticated container.

I encountered this behavior when the ChatGPT Windows desktop application initiated browser-based authentication. Firefox selected my existing account container without asking me.

The same behavior was reproduced independently using PowerShell.

Expected behavior

External links should open without a container unless the user explicitly specifies otherwise.

Alternatively, Firefox could ask which container to use.

An external application should not unexpectedly trigger navigation within an authenticated browser context merely because Firefox guesses that it is convenient.

Privacy concerns

Container isolation is an intentional privacy boundary.

Automatically selecting an authenticated container can:

• Cause unintended account reuse during OAuth or SSO authentication.

• Undermine the user's intended separation between applications and online identities.

• Interfere with Temporary Containers workflows.

• Produce behavior that users may incorrectly interpret as applications having access to their existing sessions.

I am not claiming that Firefox directly exposes cookies or authentication tokens to external applications. My concern is the automatic reuse of an authenticated context without an explicit choice.

Existing workaround

I found that the preference

browser.link.force_default_user_context_id_for_external_opens = true

corresponds to the checkbox "Don't use containers for links opened from external apps".

However, both were disabled by default in Firefox 160.0.

I understand that Bug 1874599 introduced this opt-out mechanism, but I believe the default itself deserves reconsideration.

Requested change

Please make the privacy-preserving behavior the default:

External application links should open outside existing containers unless the user explicitly requests otherwise.

Users who prefer automatic container selection could enable that convenience themselves.

Privacy should be the default, not an opt-in.

P.S. Make one setting basic_privacy = true. 🙂

[Jon]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.