Firefox 126.0.1 displays a very long name of a container completely in the URL bar. Firefox 127.0 truncates container names AWS long account and role names make keeping the long name very convenient (works with the 'AWS SSO Containers' Add-On

More information here https://github.com/mozilla/multi-account-containers/issues/2641

Thank you for the Github link. It sounds as though this was intentional to ensure that the URL wasn't obscured.

The restriction is in a built-in style sheet, any characters beyond 8em in width are trimmed:

https://searchfox.org/mozilla-release/source/browser/components/contextualidentity/content/usercontext.css#111

If this is a big problem, you could consider delving into the unsupported realm of userChrome.css rules to override built-in rules. In this case, you could use:

#userContext-label {
    max-width: unset !important;
}

The effect is visible in the attached screenshot (which obviously shows a lot of other modifications). If you are new to userChrome.css and want to learn more, I have a site here:

https://www.userchrome.org/

Note: The How To videos are missing the last step about the about:config change.

This works great, thank you !

Where precisely in firefox do I find the stylesheet to override it? thank you!

Where precisely in firefox do I find the stylesheet to override it? thank you!

You don't need to change the built-in style sheets to override them. Instead, you create your own file named userChrome.css to override the built-in style sheets.