I changed the Chromium browser (as a masters project) to intercept suspicious extension actions like inserting elements etc and to alert users of what the extension is attempting to do. Using this proof-of-concept browser would have helped you debug your ad injection problem!
One of the reasons the Chrome browser uses one process per tab is so that memory between web-pages is isolated. By doing this, an attacker in control of a renderer process cannot read data from another renderer process (web-page).
It seems like Firefox has made a security / memory tradeoff here since renderer processes can render multiple (4 by default) web-pages at a time.
But the cost of incorporating is really tiny isn't it (£15 in the UK where I live)? And I'm thinking about what users think seeing an app created by a person vs a company
https://cypher.codes/writing/intercepting-suspicious-chrome-...
- Note: my project specifically tries to protect users from Facebook hijacking and ad injection attacks - the two most common attacks on the CWS!