Thats not a UBBt problem. Threads works very well with Firefox. On your server is somthing special installed. I get the following error:
Zend Optimizer requires Zend Engine API version 20050606.
The Zend Engine API version 20021010 which is installed, is outdated.

I guess its a problem of your Server. I work with Firefox (1.07 / 1.5) on my and other UBBt page every day without problems.

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen