xdg-desktop-portal-gnome again?
( if it exists remove and replace with xdg-desktop-portal-gtk )
One thing to do would be read the update announcements thread:
[Stable Update] 2023-07-10 - Kernels, Plasma, Gnome, LibreOffice, Pipewire, Mozilla, Wine - #2 by philm
Or the forum in general:
Search results for 'firefox slow start order:latest' - Manjaro Linux Forum