Most essentilal add-on Rated 5 out of 5 stars
I've been using it for years now I think, and was usually happy with. However, if the startup time of firefox is really ~ 20-30 seconds due to Adblock Plus (it is on my system), as stated in some comments, this would be the only thing to improve, but even if it is the cause, it is acceptable.
As said, don't forget to also install Element Hiding Helpe, and aditionally, OptimizeGoogle is also very useful (95% of ads in Google seem to be fraudential/spam-like). Some really annoying ads can be blocked by using the hosts file on your computer, by adding for example
Oh, and I usually block ads that are either fraudential, or just annoying or useless anyway. If you ever changed the channel when an ad came on TV, you shouldn't have a big ethical problem with it. Everyone (usually) has to pay for internet access anyway, and there are and probably will always be lots of great websits without any ads. And there are still some interesting ads I look at.