Comment 2 for bug 299420

Third party extensions can sometimes cause bad behaviors in Firefox. It seems like you have multiple dictionary extensions that might be causing the issue. Please try this with a new profile to determine if it's a Firefox problem or due to your extensions. You can then add your extensions back one by one until you discover the one that is not working.
https://wiki.ubuntu.com/MozillaTeam/Bugs?action=show&redirect=DebuggingFirefox#Try%20with%20a%20new%20profile

Please let us know what you find out.