Like this, but i'll use firefox in this example:
Code:
LIBGL_ALWAYS_SOFTWARE=1 firefox
But yes paste that line in the terminal.
For Files (nautilus)
Code:
LIBGL_ALWAYS_SOFTWARE=1 nautilus
If you are using GTK on very old hardware, you may be better off with the old GL renderer, since it makes fewer demands on the GPU. You can override the renderer selection using the GSK_RENDERER environment variable:
"GSK_RENDERER=gl"