This is on Windows?

Usually that wouldn't happen unless another browser ran and made itself the default. Each browser has a way to prevent that from happening, so you might check what is installed and switch off the feature there in case that's the cause.

There might also be some utility/security software that prevents changes to browser settings at the system/registry level. For example, if this problem only occurs when restarting Windows, it could be something that runs at startup.

It is W7 and ff38+. the problem is, it is my very technically challenged wife's computer. I have no clue what she is doing or not. I just want to stop FF from losing default for whatever reason. OK is set IE not to ask. Maybe that will solve the problem. thx

I guess the real problem is that a few programs seem to think ignoring the default browser designation is fun & games. If that is the cause I don't know which program she is using that does that. If that was the cause then she is blindly clicking yes - well most of what she does is blindly.

anyway......... thx