When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Clear Firefox's Cache

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab > Offline Storage : "Clear Now"

2. If needed, delete the site's cookies here (this will log you out)

While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"

Then try reloading the page. Does that help?

Some sites use a Flash-based uploader. Have you noticed any other issues with Flash (audio/video issues)? There are some compatibility issues between the latest Flash player and some Firefox add-ons or settings. Could you see whether anything in this article helps: Flash 11.3 doesn't load video in Firefox.

You might also want to try changing the SPDY setting as described in this thread: Attachments fail in Gmail.