Epiphany extensions are similar to Mozilla Firefox extensions: they are entirely optional and change the way the browser works (usually by expanding its feature set). For example, an Epiphany extension could provide mouse gestures, track your browsing behavior, or even replace the entire Bookmarks system (though such an extension would be rather complicated).

Epiphany extensions are different from Mozilla Firefox's because they are implemented in C or Python as opposed to Javascript. This gives enormous flexibility and a bit of added challenge, but it should be no problem for even a beginner to develop an extension which crashes Epiphany on startup.

Epiphany extensions are at a lower level code-wise than Mozilla Firefox's; there is no simple way to convert a Mozilla Firefox extension to an Epiphany one. This should be repeated: Epiphany extensions are not interchangeable with Firefox extensions.