LXC and LXD are system containers, which are similar to VMs but using Linux container facilities.
There is an entry-level guide at https://blog.simos.info/how-to-easily-run-graphics-accelerat... that describes how to setup a LXD container so that the GUI applications in the container appear on the host. Here, the GUI applications in the container are using the same X11 server as the host, therefore there is no effort for security isolation.
It is possible for those that are interested, to use features from `x11docker` (second X server, xpra, Xephyr) to provide security isolation with LXD containers.
For the Firefox use-case that you describe, you can setup Firefox and then take a snapshot of the container (`lxc snapshot ...`). Every time that you want to run Firefox, you can switch the container back to the snapshot state and start Firefox.
There is an entry-level guide at https://blog.simos.info/how-to-easily-run-graphics-accelerat... that describes how to setup a LXD container so that the GUI applications in the container appear on the host. Here, the GUI applications in the container are using the same X11 server as the host, therefore there is no effort for security isolation.
It is possible for those that are interested, to use features from `x11docker` (second X server, xpra, Xephyr) to provide security isolation with LXD containers.
For the Firefox use-case that you describe, you can setup Firefox and then take a snapshot of the container (`lxc snapshot ...`). Every time that you want to run Firefox, you can switch the container back to the snapshot state and start Firefox.