I have configured Firefox to open eleven tabs when it launches. Intermittently, after Firefox has been open for a while, I find those eleven tabs duplicated so that I have twenty two tabs with the extra tabs in the same order as my startup tabsa. It just happened a minute ago. I have no idea what causes this to happen. I don't think it's anything I did. I'm on MacOS Big Sur 11.2.3 and Firefox 87.0. The last time it happened, I think I was on Catalina.