I launched a Firefox extension 18 months or so ago.
In the first six months or so we had a few thousand installs. In the year or so since getting up on Mozilla, we have had about another 15,000. Getting on the Mozilla site is key to drive higher-volumes of installs.
Apart from providing a readymade audience, Firefox also is much stricter on users tring to install non-Mozilla addons - meaning that the conversion rate from users *trying* to install to success is very low.
Feel free to ask away with any other specific questions.