Firefox 7.5.x also generates an error message when entering start date, end date, actual effort and percent complete saying "%complete cannot be 0 if actual effort is > 0". Does not happen in IE 7 and with Firefox 3.6.
The bug with Firefox 3.5.x (not 7.5.x) is logged with defect no. #QCCR1L24088. As usual there is no estimate when it will be fixed. For the few users we have that use firefox, I'll have them install 3.6.