Just to add some perspective to what is being asked. These are 2 sites that have a majority of the extensions available for downloading.
http://texturizer.net/firefox/extensions/ (main/official one)
http://www.extensionsmirror.nl/index.php?act=idx (good secondary source)
Just look at the list from the main extensions site (texturizer.net link) and you can already tell that it's not feasable to have all the extensions built into it.
Also not all extensions are compatible with each other. Several also do similar things and can cause browser instability.