This is exactly the reply I was expecting. The "yes it works but here's some extra things you need to do to make it work". No average user is going to do that.
Last week I was at a project meeting cum conference, where most people attending were scientists, but not computer people. They have showed what they did with computers and programming languages, and it amazed me with no end.
These people wired tons of plugins on their browsers, VSCode and Obsidian installations, created web sites and tools which broke new ground and their tools were running on distributed systems at very good speeds. Some of these tools requires human years to develop even by “computer people”, yet these people sat with their partners during the pandemic and developed these amazing things while wearing trainers and drinking coffee casually.
These people would make this plugin draw circles up in the air while most of us “not average users” are reading the docs.
So, what is an average user? By your definition I’m an average user, because I use containers, but I didn’t use this extension, but found it in five minutes, because you wanted a solution.
Now I’ll install and use it fully tomorrow morning, before my coffee gets warm. Because it’s worth it.
It's available at https://github.com/honsiorovskyi/open-url-in-container and Mozilla Extension store, so you can directly install it.
I think you can register a new "application" to open links, not set at default, and use Right Click -> Open With -> Firefox (WORK CONTAINER).
Will that work?