[6][7] Early versions of Firefox supported themes that could greatly change the appearance of the browser, but this was scaled back over time.

[3][10] Prior to 2017, Firefox supported extensions developed via various APIs: XUL, XPCOM, and Jetpack.

[16] Starting with Firefox 40, Mozilla began to roll out a requirement for extension signing.

[1] In contrast to mozdev.org which provides free hosting for Mozilla-related projects, the add-ons site is tailored for users.

By default, Firefox automatically checks the site for updates to installed add-ons.