I happen to be a devout Firefox user, and ever since I've gotten it I have needed to use extensions to make my browsing experience better.

Where?
update.mozilla.org/ or extensionroom.mozdev.org/ or extensions.roachfiend.com.
Or try a google search.
How?
update.mozilla.org is automaticly on your allow list for installing software. To add a site to your allow list, click on tools/options/web features/allowed sites. Allow whichever site that you want to allow to this list. NOTE: Never allow a questionable site to install software on your computer.
Extensions to get
The extensions that I would highly reccommend would be the following:
Adblock
YOU MUST GET THIS EXTENSION. This extension allows you to delete any image or java or flash application that you do not like. You will never see these items again. You can also set filters, such as */ads/* or */banners/*. Adblock download
bugmenot
Ever get sick of having to register (for free) to a site to access some of it's features? Examples of this can be seen in newyorktimes.com. Bugmenot allows you to right click in a form and select "Bugmenot" to search the bugmenot database for a user who has decided share his screenname and password with the public. Bugmenot download
StumbleUpon
After you have been on the internet for a while, it may get boring. You may think that there aren't any cool sites left out there. Well you are WRONG. Fill out a quick form that asks you what you like, and click stumble and you are sure to find a cool site! Stumbleupon download