When the hell will firefox have a decent profile manager?? It's the one thing that is preventing me from switching over. No, container tabs are NOT it.
The fact that GitHub don't provide a better solution here has to be actually costing them money with the network usage and extra agent time consumed. Right?
Plenty of marketplace actions will install things and/or mutate the runner. It's a matter of time before someone does something or there's a build that doesn't cleannup after itself (e.g. leaving test processes running) that ruins the day for everyone else.
The trick is to do these sort of engagements as a fixed price contract. You could have probably done an assessment in a hour or two. Then you charge on what you estimate is the value to the company.
If you are interested in having self-hosted auto scaling github actions runners but don't want to manage your own webhook handling infrastructure, https://cirun.io is worth a look.