First you need a little XML like;
Filename: sitepoint.src (put in somewhere like http://localhost/sitepoint.src)
Next a touch of HTML;
Install Sitepoint Search Plugin
Put that somewhere one your local server as well, view with Firefox and install, then you should be able to select Sitepoint from the search drop down (top right in default Firefox theme) and enter searches directly.
Judging from this part of the configuration file;
Looks like POST is supported as well – should be possible to add searches for specific forums as well, but haven’t tried.
Further documentation can be found here: http://mycroft.mozdev.org/deepdocs.html