So, Mozilla has released an experimental program, Jetpack, that allows anyone who can build a web page to build a Firefox add-on. This means that if you only know HTML, you can build simple extensions for the popular browser. Jetpack also supports CSS design and Javascript.
And for those of you who are not programming nerds, Jetpack makes one very nice change: you won't have to restart the browser to install extensions built through Jetpack.
The Jetpack experiment is a very early open-source project - in fact, it's version 0.1 - and thus, is highly buggy. But in its current form, it allows anyone to use an API to build statusbars, work with tabs, and more. It also supports external APIs like Twitter, so you can build simple Twitter apps for Firefox.
So if you're a developer, check out Mozilla's introductory video on how to build an add-on for Firefox. And if you're just a fan of Firefox who hates restarting his or her browser, just be patient: Jetpack is still a baby, and it will be some time until restarting the browser is a thing of the past.