Can't click button of firefox when I run it within firejail + xephyr

I run firefox with the following command:

firejail --x11=xephy firefox

----- The command usually opens a firefox window (800x600) that fills the xephyr window (800x600). In this case, the following problems occur. Problems: - When I click on the hamburger menu, it shows the menu briefly appears and then quickly disappears. - When I type "alt" the menu bar at top appears. Then when I type f, the file menu briefly appears then quickly disappears. ----- However, after I run the command, I can intentionally kill xephyr (by pressing alt-f4). When I run the command again, firefox starts in safe-mode. The firefox in safe-mode would open a window that is smaller than 800x600. In this case, the problem disappears ----- Somehow setting the height and width of the firefox command as below doesn't solve the problem.

firejail --x11=xephy firefox -width=700 -height=500

----- The window environment is debian 10 + jwm + xorg. I start it by startx -- -nolisten local.