Lauren Weinstein has a broader warning: While the browser firms work extensively to build top-notch security and privacy controls into the browsers themselves, the unfortunate fact is that these can be undermined by add-ons, some of which are downright crooked, many more of which are sloppily written and poorly maintained. Ironically, some of these add-on extensions and apps claim to be providing more security, while actually undermining the intrinsic security of the browsers themselves. Others (and this is an extremely common scenario) claim to be providing additional search or shopping functionalities, while actually only existing to silently collect and sell user browsing activity data of all sorts.
Lauren also warns about sites that "push users very hard to install these privacy-invasive, data sucking extensions" -- and believes requests for permissions aren't a sufficient safeguard for most users. "Expecting them to really understand what these permissions mean is ludicrous. We're the software engineers and computer scientists -- most users aren't either of these. They have busy lives -- they expect our stuff to just work, and not to screw them over."