Follow these steps:
Go to Budgets and click on the budget name (/cgi-bin/koha/admin/aqbudgets.pl?budget_period_id=1)
Click 'edit' to the right of any fund
The url of the page changes to cgi-bin/koha/admin/aqbudgets.pl?op=add_form&budget_id=1&budget_period_id=1 but the screen does not change to the edit screen
I am on Mac osx and FF4
This works in the pre-TT version so marking it as blocking 5917.
Fixed by the fix for 6208, please test
Tested on master, works now!