for Blink based Opera, there is an extension called "Filter request headers", that will allow users to block the referrer. The page offering this extension tells me, that the extension will access my data on all web sites. This i find to be somewhat disturbing, as it could mean that the extension might be accessing my log in credentials for online banking and other private data. As a user, i don't get any information on what the extension will do with the data it accesses. A worst case would be, that the extension will convey the data it has accessed from web sites i visit to its author. And even if it merely accesses the URLs of the web sites i visit and conveys them to the extension author, i still wouldn't find that acceptable.
No one in his or her right mind will accept an extension that conveys his or her online banking credentials or bank account balances to an extension author. And no one in his or her right mind will accept an extension that conveys the URLs of the sites he or she visits to an unknown extension programmer.
The information that users get on extensions' permissions and what extensions will do with data they might or might not collect are too vague. If an extension will access my data on all web sites, but will refrain from conveying such data to other parties, please make a clear statement to that effect on the page offering the extension.