> I'm trying to make a backup of a wbb2 board (woltlab
> burning board 2.0), which uses php. It works, but it
Httrack does not make a backup of a site, it makes a mirror, a static copy. 
Backups include the databases and the server side code including php, asp,
jsp, cgi, etc.

> saves the pages in php format which makes it
That is impossible.  The only thing you can get out of a web server is HTML
and associated files.  php is server side code that generates html pages.

> Whenever I go into one of the php pages Firefox
> autofires window after window after window until it
> either crashes or I terminate it in the task
Try httrack-3.42-3.exe -
<http://download.httrack.com/cserv.php3?File=httrack.exe>