The thumbnail generated as a preview on the upload form, on any Wikimedia site, causes Firefox to hang until it is created. With large files, especially SVGs, Firefox will be unresponsive.
If you select a large JPG to upload via http://commons.wikimedia.org/w/index.php?title=Special:Upload , there is a noticeable delay before the preview thumbnail appears in the source file box. If the file is a large SVG the delay is *much* longer. Firefox will not allow completion of the upload, or any other activity, until the thumb is generated.
OS: Windows 7