Tinymce edit/insert image pop-up not always in FF

[Gregor]

Hello,

Just upgraded two sites to v1.3. The weird thing:
- site A www.uisge-beatha.eu in FF; the insert/edit image screen pops-up however it does not show any buttons
- site B www.wsv-herkingen.nl in FF; the insert/edit image screen pops-up and it does  show the buttons
- site A in Safari; the insert/edit image screen pops-up and it does show the buttons
- site A in Safari; the insert/edit image screen pops-up and it does show the buttons

Could anyone give me a suggestion where to look? I checked the tinymce setting, on both sites they are equal. FF allows javascripts.

thanks in advance for your reply!
Gregor