Hey, but you can! ;)

In the options page, below the list of engines, you can see "Load browser search engines". When you find the file with Firefox's search engines on your machine (there are instructions right beside the button), SSS can import them.

NOTE: SSS could use the Firefox search engines directly (and it may in the future) but that's more limited since it wouldn't get access to the search urls and the way the tabs would open would be controlled by Firefox, not your SSS settings.

Cheers!
Daniel