Why is there a warning about x-notifier ?
Puzzling Firefox warning
After updating x-notifier on Firefox 40.0 the Add-on list contained a warning: X-notifier could not be verified for use in Firefox. Proceed with caution.
On the explanation page it states:
https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox?as=u&utm_...
Where would I encounter unsigned add-ons? Add-ons installed through the official Firefox Add-ons site undergo a rigorous review process before they are published. These add-ons are signed and verified. When you install an add-on through another website, Firefox checks to make sure that the add-on has been digitally signed before you can install it.
X-notifier 3.5.12 is on the official site:
https://addons.mozilla.org/en-US/firefox/addon/xnotifier/?src=cb-dl-updated
X-notifier (for Gmail,Hotmail,Yahoo,AOL ...) 3.5.12 by Byungwook Kang Notifier for gmail, yahoo, hotmail, aol and more webmails. X-notifier(aka WebMail Notifier) checks your webmail accounts and notifies the number of unread emails... Supports : gmail, yahoo, hotmail, POP3/IMAP, facebook, twitter and more
Another explanation site offers no further explanation:
https://developer.mozilla.org/en-US/Add-ons/Add-on_guidelines
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In the future, Firefox will refuse to run unsigned extensions. However, the first step being taken in Firefox 40 is to warn you that you have some so you can check for updates and/or inform the publisher about this impending problem.
Hmm, what version of X-notifier do you have? The current version (3.5.12) definitely should be signed: https://addons.mozilla.org/firefox/addon/xnotifier/
If you didn't install from the Add-ons site, I suggest installing that current version, however, I don't know what happens to saved data if you change the install source, so you might want to research that a bit first.
The X-notifier (for Gmail,Hotmail,Yahoo,AOL ...) 3.5.12 as found on the Add-ons site is signed as you can see by the presence of the META-INF folder if you inspect the XPI (ZIP) file.
If this isn't the case then you might have security software that is modifying the XPI file.
You can boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to check this.