You can not customize it barely at all, but
Forumer.com has good free Invision Boards.
Freewebs.com isn't horrible webhosting
Also, try Angelfire- You can bypass the ads by naming all page extensions as .msnw
They may have fixed it, but last time it was working to get rid of those ads.
(May not let Firebird..I mean Firefox view it, havn't tested, should work though)
EDIT: You need near admin access to install some boards on a server (Invision is one I believe), so most Hosters provide an easy Install button. Free usually don't.
And on a side-note: The name Firebird for that great browser..

even though you were a great name.