How are you using a different proxy per container tab? I've tried FoxyProxy but it leaked DNS requests through the local network resolver even with SOCKS5 proxies.
Go into the main extension settings of 'Firefox Multi-Account Containers' that lists your containers and click on 'Manage containers'. Select any container and the last option on the new page is 'Advanced Proxy Settings'. This setting is per container.
ah, that would be way too much work for a one-off proxy switch for one site