Some extensions are Firefox-specific (only relevant for managing Firefox itself), and there is considerable overlap in functionality for several extensions. That leaves the following summary:
* 41 extensions offer out of the box functionality in Opera 9, with roughly equivalent capability,
* 13 extensions are available by installing a UserScript or a Widget or dragging a button,
* 17 extensions require advanced customization or are only partially available,
* 45 extensions are not available at all in Opera.
[updated]After the December 3, 2006 update, out of 96 sets of extensions:
* 32 (sets of) extensions offer out of the box functionality in Opera 9, with roughly equivalent capability,
* 11 (sets of) extensions are available by installing a UserScript or a Widget or dragging a button,
* 13 (sets of) extensions require advanced customization or are only partially available,
* 40 (sets of) extensions are not available at all in Opera,
* 3 extensions are are Firefox specific (like dictionaries).
[updated again]After the July 29, 2007 update, out of 113 sets of extensions:
* 38 (sets of) extensions offer out of the box functionality in Opera 9, with roughly equivalent capability,
* 14 (sets of) extensions are available by installing a UserScript or a Widget or dragging a button,
* 21 (sets of) extensions require advanced customization or are only partially available,
* 38 (sets of) extensions are not available at all in Opera,
* 4 extensions are Firefox specific.
Corrections are welcome, but only from people who have actually studied both the extension and Opera's behavior. 'why would I need it' is a valid reaction to some extensions, but will probably not lead to changes in the table
http://my.opera.com/Rijk/blog/...x-extensions-and-opera