I think it was after the last update that I had today in the morning. It was automatically updated on my iPhone. And after that, all my open tabs were closed. I noticed the same happening before as well, I am guessing it should be because of an update as well, but I am not sure of the reason. I had some 60 something tabs open and when I opened a link from an external app onto firefox, then it removed every other tab except that new tab.
Also, each time I close and open the app, almost all the websites are getting logged out and I keep having to login again. Websites like youtube, and theodinproject.