> For instance the security model for a browser should be ultra tight and protect the user from the site, but as a developer I'd want to access and modify my files directly through the inspector panel.
I don't see any inherent conflict between preventing sites from having access to the local filesystem, while having powerful local filesystem access from the browser's integrated devtools. One does not exclude the other.
> Another example would be the use of cache, where I want the minimum possible retention while a user would want the opposite.
Caching rules should be managed on the server, not the client. Properly configured servers/applications never need anyone to clear their caches explicitly. I don't see why you would need a special cache-less mode in the browser if you've set up your servers correctly.
In my view developer modes (in browser or framework) are counterproductive because they create discrepancies between how your users experience your site or app and how you experience it. Your browser should have the exact same app experience as the one your users use, so it shouldn't have any special developer-mode behavior. That doesn't mean you don't need powerful inspection tools, just that those tools should be built around that standard experience instead of altering it.
