Neowin.net sums up the essential extensions, they call them plugins, to Mozilla Firebird in this article. Now that Mozilla is moving closer to the Mozilla Firebird 0.7 release, it gives a nice idea about how the lightweight browser can be extended with about as any functionlity you want. And the nice thing is: if there isn’t an extension available between the tens and hundreds that have already been written, that suits your needs, you can write it yourself!
To the list that they give I would like to add my favorites:
– Tabbrowser Extensions: Adds about every functionlity you can imagin to tabs and the tabbar: auto-reload, drag-and-drop, tabgroups, reload on double-click, and many, many more… A great, no I call it an essential, extension by Shimoda Hiroshi
– StyleSelector: Nice extension for web developers who want to sea how their site looks with different stylesheets, or with no stylesheet at all. Multiple stylesheets are only supported by Mozilla based browsers. By Stephen Clavering.
Try them and you can’t live (read: browse the web) without them.