i fixed this a slightly different way which i find easier.
login to the backend of your joomla site.
click on extentsions and select modules.
Locate the login module..in my case i am using the mod_cblogin which is the community builder login.
Tick the box for this module and then select copy. This will duplicate this module.
Then open the copy of the module. Give it a name...in my case i named it the same as the original to give the impression there is only one login module.
Then where it says Login Redirection URL you need to point it to the URL of your fireboard forum.
Then i changed menu assignment to be select menu item and only select the forum and no other items. By doing this you will only show this login on the forum page.
Then you need to open the original login module and remove the forum page from its menu assignment so you don't have 2 login prompts.
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Since i am using community builder the login link on fireboard doesn't go to cblogin it goes to joomla login which i tried the disableregistration plugin but it didn't work. so i ended up putting a redirection on this link to point back to the forum page.
example of my site can be found here
myveryownpcguy.com/index.php/forum