X-notifier could not be verified for use in Firefox.

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Solved problem?

You need to download X-notifer from addons.mozilla.org, this warning will disappear.

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Suggest/Correct URL from jeroen

Try reinstalling from addons.mozilla.org (assuming you are on windows):

https://addons.mozilla.org/en-US/firefox/search/?q=x-notifier&appver=43....