cannot keep Firefox default browser

I purchased Dell desktop computer, Windows 11, I installed Firefox, and I made Firefox the default browser via Firefox and the computer's settings. I selected default dozens of times. Each time it stays the default for a while and then another web browser becomes default. I then again select Firefox as the default web browser, both ways, and it stays that way for a long time or a short time. A few minutes ago, after the selection, the settings window for the app page flashed and Firefox was no longer the default browser. I have Google Chrome and Microsoft Edge installed for the few times that Firefox does not work on a particular website. What gives? Thank you