I was wondering if lastpass plugin could be used with reasonable security. If not what are the security concerns? would it be more secure to visit the site? even though lastpass does require the use of javascript. How could I go about testing this plugin? or is a remote password manager always a bad idea? I have read in their documentation that the only information stored on their servers is heavily encrypted.

What I am most concerned about is the possibility of DNS leaks.

I am familiar with it but I have never used it. JavaScript is not an issue; however, I do not recommend a remote password manager for a variety of reasons.

You should consider www.keepass.info

The Add-On is closed source software, thus we won't know for sure (although there are versions without binary blobs available). I would not use it.