I was using Firefox 66.0.3 (64-bit) on Windows 10 and around 2019-05-03 21:00 EDT got a notification that some addons have been disabled. I open about:addons to see that all my addons have been disabled.
These include uBlock Origin, Amazon Assistant for Firefox and Facebook Container. They are disabled with the message: "could not be verified for use with Firefox and has been disabled"
Going to an addon page on addons.mozilla.org and clicking "Add to Firefox" gives me an error message: "Download failed. Please check your connection."
This is reported by other users: