Is there a screenshot tool that works without bugs for extremely long pages with firefox and chrome. Preferably external (software) or internal (add-on) ?
I haven't found yet an add-on that works.
I use windows 7.
My favo(u)rite screen capture tool is FastStone capture.
Among many other excellent features, I have often used it's auto-scroll feature to capture an entire browser page, no matter how long it was.
The current version costs $19.99, but you can download the last free version from http://whatsonmypc.net/2009/05/14/faststone/ Chances are that you don't need the features in the newer version
SnagIt is more expensive than other answers here (~50€), but also offers auto-scroll, in both axis if necessary. I have used it, for example, to screenshot the output of database queries. It can also easily screenshot a whole window, or just window contents, so it's easy to get a good image without having to manually crop much afterwards.