It might open up for a terrifying level of abuse, but if you can have Dtrace and eBPF implemented in the Linux kernel, you can surely design an API for allowing AIs to be plug-able within Firefox.

Firefox is already a really good browser, Mozilla really should be focusing on that. They can design and implement an AI plugin system to go into that core. People who want AI can install an agent and enable the AI sub-system. If the AI companies won't implement it, Mozilla can do it and charge a fee for the plugin.