Sorry, I don't use that screenshot program myself, but thereis a comment on the download page about another user having a problem like that due to the WMP 1.1 add-on.
https://addons.mozilla.org/en-US/firefox/addon/lightshot/

The other thing would be to contact the developer at Skillbrains.

Thanks for replying. I don't seem to have the Windows Media Player plugin, so I assume that the problem is caused by something else.

Hello, My name is Serge. I'm developer of the Lightshot. First of all I want to apologize for the inconvenience caused by the incorrect work of our add-on. We already know about this problem, but it is very difficult to reproduce it in our environment. Please try "Lightshot desktop application", hope it will work well for you. You can download it from here http://app.prntscr.com/

Thank you for responding, Serge. I've already downloaded the desktop version of Lightshot and it works fine with Firefox. Which is good because I like Lighshot very much and didn't want to have to find another screen capture program.

All the best. :}