The photoblog plugin throws this javascript error in Firefox when you click the Done button while adding a photo:
Error: document.getElementById(el).onchange is not a function
Source File: http://timf.anansi-web.com/serendipity/ ... _editor.js
Line: 142
It does insert the photo, but will not insert the thumbnail. This does work in IE and a quick Google shows me that there are problems with the getElementById function in Firefox, but I'm not a JS programmer and can't figure out how to fix it.