to do that all you need to do is open notepad and type the following
<search
name="HSX forums"
method="GET"
action="http://www.google.com/search"
queryCharset="utf-8"
>
<input name="q" user>
<input name="sitesearch" value="www.hsx.com/forum">
</search>
and save the file as an .src file. For example mine is call hsx.src
you have to save the file in the firefox searchplugin folder, usually C:\Program Files (x86)\Mozilla Firefox\searchplugins or C:\Program Files\Mozilla Firefox\searchplugins.
restart firefox and your good to go. If you want you can also use a gif pictuure that is exactly 16x16 pixels to use as an icon. I did a screen capture of my screen and used the blue circle with HSX in it. save that in the same folder with the same file name except .gif instead. so for me it was hsx.gif