Firefox is a cross-platform application which means it shares a lot of code across multiple platforms (Windows, Mac, etc.). Like many cross-platform applications, this means it doesn't always take advantage of or support Mac-specific features. This is the case with Firefox for Mac. For example, if you visit a web page and select some text and graphics on the page and then choose Edit > Copy, Firefox will only copy the raw HTML code and a plain text version of the selected content to the system clipboard. So when you look at the clipping in iClipboard, you will only see the plain text that you copied and none of the images. This is a deficiency of Firefox, not iClipboard. iClipboard is only capable of capturing whatever Firefox copies to the System Clipboard.
If you do the same thing with a web browser like Safari, Safari will also copy a rich text version of the selected content (including the images) to the System Clipboard. This means your clipping will show up in iClipboard with all of the expected text formatting, images, etc. Safari simply supports more of the data formats that Mac users expect.
Whenever iClipboard captures a clipping, you can click the the Preview (eye icon) button in the Shelf to see a Preview window. At the bottom of the Preview window is a Format button you can click to see all of the different formats that were put on the system clipboard. Clicking this button is a useful way to see the type of data that Firefox (or any other application) captures. If you like to see Firefox capture other data formats like rich text, images, etc., please contact the makers of Firefox and request that they add better support for richer formats.