This problem has been fixed in version 7.0+.
A recent Mozilla Firefox update (e.g. 3.6.9) has broken the "Insert Image" feature in the Rich-text editor. Because of this, it will appear that you are not able to insert any new images using the Rich-text editor in Mozilla Firefox. This bug ONLY effects new images, and does not effect images that are currently on the Page.
There are a few workarounds to this issue:
1) Copy and paste an existing image, and then use the "Insert Image" feature to change the image.
2) Add the "src" attribute to the image tag in the HTML view in the Rich-text editor (e.g. src="/example.png").
3) Use a different browser (ie. Chrome, IE, Safari, or etc.) to insert your images.