This extension is very good, but I use multiple steam accounts in firefox using Multi-Account Containers, it only works in the standard tab, in the other tabs using Containers the extension just doesn't work, please fix it, then I would give you another rate.
The extension won't support multiple user simultaneously. It would require a complete rewrite and would add a staggering amount of complexity. Just use different firefox profiles for each user, which this Multi-Account-Container seem to be basically, just less separated.
