So, I've just added support for OpenSearch to iusethis. This means that you can easily add iusethis search to the Firefox search box.
As you can see from the screenshot, you just press the arrow next to the search engine logo (On my machine the arrow turns red when there is something to add, but your milage might vary). Then just click the 'Add iusethis' link, and you are ready to go :)
There’s no instructions and personally I’m using Inquisitor for Safari which looks like it doesn’t support OpenSearch. The arrow just puts a cursor in the search box, nothing more.