Strange bug - windows occasionally changing style when moved

Occasionally when I move several Firefox windows from one desktop to another using the pager, one of the windows changes its properties.
The NoTitle and WindowListSkip flags somehow turn true.

Look the FvwmIdent output for such a “modified” window][/url]

For comparison, here the FvwmIdent output for another Firefox window that didn’t get its attributes changed][/url]

Any idea what could be the reason for that strange behavior?

Are you referring to a missing titlebar ?

If so,I saw that happen once (twice) in fvwm2, and also on a system at work using either KDE or GNOME, do not remember, that was a year ago or so. I pretty much suspected it was caused by a Firefox issue. Every so often Firefox would do some weird things in whatever release was prior to 31.x. So far since 31.x came out no issues.