I've been using Firefox on mobile for years now and it for whatever reason it doesn't allow you to screenshot when on a private tab. I would use normal tabs but I forget to close them.
I've been forced to open links with Chrome countless times because of this issue and I'm tempted to just switch to it permanently.
If you really want that feature in the app then please make it optional.
Update:
Apparently it is an option, it was just difficult for me to find and the developer eho replied didn't know it was an option either