golfthere --
I have been setting up some blogs here on WordPress.com over the last few days and I had the same trouble you are having with getting remote images to "green check" by loading from a URL.
The images I wanted to load are from my own private server -- so I knew the images were alive and working elsewhere -- and while I have .htaccess hotlink protection, the WordPress.com blog URLs were included as approved remote images pullers.
Here's where it gets strange: While the images would "Red X" under Firefox for Mac, they would load just fine in the WordPress.com editor using Safari for Mac and Google Chrome for Mac.
Very strange.
So, I did a lot of testing, and it came down to me having to turn OFF "Always use HTTPS when visiting administration pages" in "Browser Connection" in "Personal Settings" here on WordPress.com.
After I removed the HTTPS connection, and cleared my cache and closed Firefox -- I was able to get the "Green Check" loading remote images via URL under Firefox.
If you aren't using HTTPS -- then I'm not sure how to help you, because I tried everything else... turning off Flashblocker and Adblock Plus and a whole host of other features in Firefox and WordPress.com and on my private server... and it was WordPress.com HTTPS that was causing Firefox to choke.
Best Wishes,
db