With recent update, themes keep spontaneously changing

TB 128.3.3esr (64-bit)
opensuse tumbleweed

With this version, the theme keeps changing, spontaneously. If the program is idle for a while, it gets bored and changes the theme.

There does not seem to be any way to stop it from doing so. I have manually selected a number of different themes; none of them are permanent.

Any suggestions?

Could you please report a bug on https://bugzilla.mozilla.org/ ?