Bit of a weird question probably but it’s naging me for almost 6 weeks so…
I use CachyOS (an arch based distro) and it is great. Cachy comes with a Librewolf-Firefox based browser that is my daily driver. Never had a problem I couldn’t solve until TrueNAS came about. 
Surfing to the GUI is not a problem but when opening a subwindow (press edit on a user) the window opens as a transparant window without content. It drives me crazy. I know switching to Firefox would solve my problem but if anyone knows a solution, I’ll send good karma.
Have you checked your browser permission settings for popups are not blocking this?