I wish I had something analogous to Multifox, preferably operating on a per-tab basis.

I looked into making something of the sort, but unfortunately the necessary API calls (e.g. notify my extension BEFORE a cookie gets set) don't seem to exist, so any solution using the existing API would be a bit hackish.

EDIT: It may be possible to do at the level of intercepting all HTTP requests in the affected tabs? (No idea if the API allows that, or if it would be a good approach even if it were possible.)