It is not a Firefox problem.You may see this message in Chrome as well.
If you are really skeptical about errors and lazy to use a different browsers.
For benchmarking, when you see this message;
1- Close the current window
2- Delete History (temporary internet files, cache, cookies etc..)
3- Open Joomla Backend and try again
and you won't see any error message.
but instead of doing the things above, just switch to a different browser and the error message will be gone.
Topic is closed.
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