They’re probably A/B testing it on only part of the users. I didn’t get it for a long time either, but when it appeared the first time it just kept showing up again and again after that.
I was having really bad time with the constant adblocker warning pop-up yesterday so I disabled all other extensions, restored uBlock Origin to the default settings aswell as cleared the catche and updated filter-lists. Then I cleared all Firefox cookies and restarted it. This morning I did not get the warning unlike the few past days, so far so good. I’m sure this isn’t a permanent fix, but for those of you still struggling with it, these are the steps you might want to try.
First I only saw screenshots of others getting it. Then I got it once, cleared the catche, updated and hoped for the best. The next day I got it again. Today I’ve gotten it 4 or 5 times. I wonder what’s up tomorrow.