I don't have any experience with FireFox. I'm an old fashioned IE user :-)
Nofollow links, for anyone wondering, are hose which do NOT count towards your PageRank backlinks. Usually it's something that applies to forums where you can have a linked signature, to prevent google viewing lots of links from the same or similar URL as spammy. The code is built into most forum interfaces that have this option.
However, there is no add-on that can possible have any effect on Google. This is because 99% of other page viewers other than you will not have the same add on.