On a new fresh installation of manjaro i3 windows manager, I had this issue where the open/save file dialog was showing a blank screen. Upon investigating, I found out that the dialog box was just too big because of my resolution hence was filing up the whole screen.

I can confirm my resolution with the following commands

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xdpyinfo | grep -B2 resolution
xrdb -query | grep dpi

Query to check the handler

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xdg-mime query default inode/directory

Upon checking i3 logs I found the following:

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[..] window 0x014013e1 wants to be at -1670x-1466 with 6220x4644

The solution is to add the following config to i3wm config

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floating_minimum_size 500 x 300
floating_maximum_size 2000 x 1500
for_window [class="(?i)firefox" instance="^(?!Navigator$)"] floating enable