Having done so I got support threads that people couldn't upload with FF anymore. IE and Chrome, no problem, but when users clicked the Upload button in FF there were no files shown.
I thought it might be baulking at the .mobi extension so I removed it and, hey presto, the files re-appeared. However, as a test, I added a different extension and once again the files disappeared from the right-side file pane.
Note: You need to do a page refresh after changing the extensions in the XF ACP!
After experimenting it seems that with 17 extensions FF will show available files for upload; but with 18 the file window is blank (even if there are qualifying file types).
Would someone please be kind enough to test this on their own sites to confirm?
Even if it turns out to be a FF issue, I'd still like to rule out XF and my add-ons!