The funny thing was I discovered that signing/expired certificate issue when I read
Mozilla's blog (my habit, I did it on regular basis) and I checked the addons for my Firefox (66.03) and nothing was out of ordinary. I was like, really, so what is the problem, because there is nothing to see here. The moment I went back to read the blog, Firefox immediately warned me that all addons have been disabled. I checked the addons and yes, they were disabled. All of them.
I fixed the issue by installing the .xpi hotfix issued by Mozilla (the same one
@Regeneration posted before) although I got it from one of my friends who got the hotfix through Studies. I disabled Firefox data collection so I could not use Studies to get the hotfix. All my addons were enabled instantly as soon as I installed that hotfix long before Mozilla issued an official update (66.04) for users who disable Firefox data collection (and Studies, since it's part of the data collection). Although it worth mentioning that xpinstall.signatures.required in all my Firefox (66.03 and 56.02) is always set to false, not sure it helped or not. Well, at least I didn't have to wait for official fix or update or enable data collection or Studies.