For the best accessible experience with Orca or Cthulhu some variables need to be exported when X or Wayland starts. Depending on the display manager you use, these variables might go in different places. There are 2 places I tend to store them, ~/.xprofile, and in /etc/profile.d/accessibility.sh. Here’s the list I have for now:

# Accessibility variables
export ACCESSIBILITY_ENABLED=1
export GTK_MODULES=gail:atk-bridge
export GNOME_ACCESSIBILITY=1
export QT_ACCESSIBILITY=1
export QT_LINUX_ACCESSIBILITY_ALWAYS_ON=1
export SAL_USE_VCLPLUGIN=gtk3

Some distros include these by default, so you will only need to do this if the variables are not already exported somewhere. To find out if a variable is set, open a terminal emulator, mate-terminal, lxterminal, etc, and type the following:

echo "$ACCESSIBILITY_ENABLED"

If you get the number 1 as output, it is set. If, however, you get a blank line, it is not set.