Если открыть кучу вкладок в FF, падают X, в Xorg.0.log такие сообщения:

[  1233.424] Failed to compile VS: 0:13(43): error: `pos' undeclared
0:13(14): error: operands to arithmetic operators must be numeric
0:13(13): error: operands to arithmetic operators must be numeric

[  1233.425] Program source:
#define ATLAS_DIM_INV 0.000976562500000000
attribute vec2 primitive;
attribute vec2 source;
varying vec2 glyph_pos;
uniform vec2 fill_offset;
uniform vec2 fill_size_inv;
varying vec2 fill_pos;
uniform vec4 v_matrix;
void main() {
       gl_Position.xy = primitive.xy * v_matrix.xz + v_matrix.yw;
       gl_Position.zw = vec2(0.0,1.0);
       glyph_pos = source.xy * ATLAS_DIM_INV;
       fill_pos = (fill_offset + primitive.xy + pos) * fill_size_inv;
}
[  1233.425] (EE) 
Fatal server error:
[  1233.425] (EE) GLSL compile failure
[  1233.425] (EE) 
[  1233.425] (EE) 
Please consult the The X.Org Foundation support 
         at http://wiki.x.org
 for help. 
[  1233.425] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[  1233.425] (EE) 
[  1233.425] (II) AIGLX: Suspending AIGLX clients for VT switch
[  1233.449] (EE) Server terminated with error (1). Closing log file.

C другой стороны похоже, что эту проблему уже исправили - https://lists.x.org/archives/xorg-devel/2017-February/052781.html

У нас слишком древний Xorg?