Just a minute ago, I was browsing the internet and typing as usual, and I got a little popup, as shown in the image.
A few days ago, on regular Firefox at work, I'd accidentally lost my tabs through misclicking to close out the whole browser window. So given I have recently had the annoyance of losing my tabs, I'm quite grateful I was saved from making a similar mistake today!
I hate this feature. Chrome does it too.