I had earlier posted this question to superuser. Got a comment suggesting that I ask the question here.
What I want to know is, where can I find trusted and open (in the foss sense) security reviews of chrome/firefox extensions.
No. "Where to find a resource" questions are generally off-topic across most of the SE sites, because the answers are simply links to an external site.
To answer your question, you won't find what you are looking for. Any security review becomes obsolete once the code is updated. Closed source code reviews will be posted by the vendor (and then only as a snapshot in time). And there is little commercial benefit for someone to take on the potential liability for reviewing FOSS code and declaring it in a certain state or not.
Security reviews are useful for the vendor, but they introduce liabilities for all involved. So, they tend to be kept private.