Yes, same problem for me today, using FF 66.0.3. Today after an hour of using FF, it decided to deactivate all add-ons.
Trying to download the latest version of the add-on just says “Download failed. Please check your connection”. Opening the link in a new tab, the real reason is shown: “Firefox prevented this site from asking you to install software on your computer”.
Except that the site is already whitelisted. I tried to disable basically all protection in the Options, as well as the XPI options in about:config. Even though that would be a bad idea, it should allow any site to install, but it still blocks it.
I put the link into Chrome in order to download the XPI, but FF claims it is corrupted.