Thank you for providing a public site to reproduce the issue.
In order to make this to work in Safari please take the following steps:
1. Check the IsUrlPartial checkbox
2. Change the PopupUrl property from:
3. Change the ModalPopupPartialCaption from:
"A Modal Pop-Up -- Webpage Dialog
"A Modal Pop-Up"
Unfortunately, the issue with Firefox turns out to be a bug. I logged it in our PITS system and you can track its progress and vote for it here: Public URL
. Since there is currently no workaround, we'll do our best to fix this ASAP.
I have updated your Telerik points for the caused inconvenience.
All the best,
the Telerik team