This is a feature request, feel free to close / redirect me if there's a better place to do this.

For reference, the Selenium Ruby code no longer seems to work for using Firefox::Profile#add_extension. I see that there's an endpoint to do this now, but this really seems like something that belongs in the capabilities information, not changed after opening the browser.

Ideally we wouldn't need "complicated" code to create a whole profile, we should be able to just encode the extension and pass it along in the capabilities like this:

{
    "capabilities": {
        "alwaysMatch": {
            "moz:firefoxOptions": {
                "extensions": [base_64_string_extension_one, base_64_string_extension_two]
            }
        }
    }
}

Alternatively, we (I) need to figure out how to fix the Ruby code. :)