I was trying to play minecraft in the browser, and I got asked to install a plugin called "IcedTea-Web Plugin (using IcedTea-Web 1.2 (1.2-2ubuntu1))" (something related to Java 6), which I can see now in Tools > Addons > Plugins. There was another plugin to install (for Java 7), I didn't know if I had to select one, so I just clicked Install or Accept.
Now the game or java app only shows a black screen, so I thought I should uninstall the plugin and try installing the other one. But there doesn't seem to be a way to uninstall plugins through the firefox GUI.
Any ideas?
As per my comment on the question, mozilla provide some instructions for manually uninstalling.
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This will help you:
This solve your problem.
note:
If you didn't get the menu "Recent Updates", follow this for step 3, click on the icon near the search box, the icon has an image of screwdriver, in that you will see "Recent Updates".