I use Firefox exclusively and I have seen this message but my observation is: 1. This appears only an account that is not used frequently. 2. This never appears

I use Firefox exclusively and I have seen this message but my observation is: 1. This appears only an account that is not used frequently. 2. This never appears on an account that is used frequently. 3. Even on account used in-frequently, say once in 2 months, this appears at most once in a year. 4. The bug may be in Google's scripts or even in Firefox, it's hard to say.