Not much you can do, user agents continue to act as agents for the users, meaning you can serve them stuff but beyond that it's up to them to dictate their experience, for better or worse.
It really sucks when extensions do fudgy stuff in global space and sometimes break your stuff though, agree. Best approach I've found is to have a help page you can link to so people can go through the typical steps of "disabling all extensions, clearing cache, etc, etc" when things break in very unexpected way and you find no causes for it.
