It's already been fixed on here (and is unrelated to the browers being used) by turning off the combined fluid view on the forum (it was in the "Community Managers ..." forum, not in this one, but the way)
Easy to reproduce:
Create a forum
Create a sub-forum in that form
Back to the first forum - go to display settings and enable combined fluid view in the display settings
Go to the front-end and view the forum - click 'start new topic' and you get the dialog with 2 forums to choose from
Back to the ACP - delete the sub-forum
Back to the front-end, refresh the forum - click 'start new topic' and you get the dialog with only 1 forum to choose from
Back to the ACP - edit the forum, look at the the combined fluid view setting and it is off (but it isn't really...click save and now it is)
Back to the front-end, refresh the forum - click 'start new topic' and it now goes to the form as there is only one forum available, so no dialog.