Hi!

I use the official archlinux docker image for this.

I'm using Arch Linux and I'm wondering how I change things and add addons in Firefox in a docker container?

eg. I need to change the language to US English but that needs to restart Firefox and that ends the docker session and won't save it.
Is there a way to start that docker image and inside that start Firefox and do all this? As far as I know that needs a lot of graphic things to be added and as this image is just to be used 'externally' to the host's OS.

oh and this can also cover all scenarios with like anything in a container

Docker is ephemeral. Changes won't persist unless you can put the changes in your external folder that it loads. Typically changing data you would need to rebuild the docker to contain these things.