Many Firefox users around the world have lost their usual set of extensions due to their sudden shutdown. The event occurred after 0 UTC (Coordinated Universal Time) on May 4 - the error is related to the expiration of the certificate used to generate digital signatures. In theory, the certificate should have been updated a week ago, but for some reason this did not happen.
The same problem happened almost three years ago, and now, speaking to Engadget, product leader Kev Needham said: Firefox. We know what the problem is and are working hard to bring this functionality back to Firefox as soon as possible. We will continue to provide updates through our Twitter channels. Please be patient while we fix the problem."
There is currently at least one workaround, but it can only be used when using the Firefox Developer version or earlier builds of Nightly. If you look in the "about:config" section and set the xpinstall.signatures.required parameter to False, then the extensions will start working again.
If you are using a different version of Firefox, there is a way to temporarily fix the problem, but the user will have to repeat it every time they open the browser. It provides a debug mode for extensions and manual loading of .xpi files for each of them.