Any extra code that you attach, gives an additional chance at security issues. Especially if you install an addon that the developer is no longer actively supporting, you might end up with issues that aren't even known until something goes wrong.
On the other hand, they are just so useful, so people use them anyway.
It doesn't matter what browser your using or any added security feature you may use , if the powers that be in your country want to know anything about you they have access at the drop of a hat... But 99.9% of law abiding people need not worry, ever.. eMails or sites visited are ALL kept in logs by your ISP... Well here in the UK that is the law , cant speak for other countries...
Go for a browser you feel comfortable with or maybe 2 or 3 of them... each have a different niche ..
I use Comodo Dragon , Torche both Chrome based & Comodo Ice Dragon Firefox based..