Firefox uses open source coding, thus allowing people to write their own scripts, making the customisability of firefox endless, you can find everything from web designing tools to media management tools.
Firefox has its own built in firewall, and thus making the pop-up blocking, which is already minimal due to the number of people that use firefox compared to IE, virtually none.
Some capabilitys are:
* Switch user agents and pretend I'm Googlebot or Yahoo's Slurp to detect cloaking.
* Easily read an rss feed.
* Pretend I'm a user in Argentina to see if Google serves up the Polar Bear Logo down there too.
* Disable the referer info when browsing.
* View all the http headers a server returned when I clicked on a link.
* View the source code for a site in a neatly formatted text editor that's color coded.
* Show all the anchors in a page.
* Show all the heading tags in a page and in the order they're rendered.
* Validate a page's css.
* View a page's style sheets.
* Disable cookies, java, images, etc... all with a single right click.
* Save specific sessions / visits on a web site.
* Delete my downloads after viewing them.