If the add-on isn't working for you, try these steps
This Version isn't allowed on Mozilla AMO due to using eval even though the use isn't dangerous. You can download it from the releases here. The code for this is open-source so if you're doubing that the code is safe feel free to look. If this doesn't help you, read on!
This is for when you're suddenly back to new twitter or never got back to old twitter. For other issues check known issues below or submit a new issue.
First, try hard refreshing twitter. Shift+Click on reload in the twitter tab.
Next, try clearing cookies for twitter. Click on info, then WAIT for the clear cookies button and click it. Now just click remove. You'll have to log back in. That should fix most issues.
If that still didn't work, try reinstalling goodtwitter by going to the store page, clicking remove then installing it again.
One issue could also be Firefox not being updated. Click on menu, help, then about, then make sure firefox is up to date.
If all that didn't work create an issue on GitHub here
Copy and Paste should be working for text. It won't work for images unfortunately, I don't really know how to fix it. Refer to issue #4.
This happens sometimes. Shift+Click on reload and it should be fixed. Refer to issue #9.
No fix yet. Issue #6.