Did you read Netcraft's press release: Netcraft releases Heartbleed indicator for Chrome, Firefox, and Opera ?
And what it says under Update: at the bottom?:
'Several readers have noted the privacy drawbacks posed by use of the Netcraft extension. Among other things, according to one reader, the extension sends vistited URLs
that can include query strings, and it does so using an unencrypted HTTP connection. If true, that leaves a list of visited sites open for surveillance and man-in-the-middle attacks. Netcraft has a fuller privacy disclosure in the "About" tab of the extension. Ars readers are encouraged to think over the implications before installing the extension.'
The reader comments can also be found at the bottom.
On a personal note. I haven't had much time to read more about it. I'd like to gain more knowledge before testing it.
What I do know for sure. Netcraft isn't the only site offering the download. If anyone is going to give the extension a try, I strongly suggest being mindful of the site it's downloaded from.
Now there's an easy way to flag sites vulnerable to Heartbleed =>