This should work if you paste the image in a rich text editor area that accepts HTML code (clipboard: image/jpeg or image/png).

Websites often use a script to intercept paste, figure out what kind of data is incoming, and then do special things with it. Sometimes these scripts are not tested in Firefox, or are written for old versions of Firefox using methods that aren't supported any more. It's difficult to generalize.

What is the site that has this problem?

jscher2000 said

What is the site that has this problem?

Twitter. I use the GoodTwitter add-on, though it still happened before I used it.

Also, it works on other computers, so I'm pretty stumped here.