X-notifier could not be verified for use in Firefox.
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Proceed with caution. (Warning Banner)
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....