I had the same problem in firefox 40.0.3 if I set "layers.offmainthreadcomposition.enabled" to false in about:config, then I can also set gfx.xrender.enabled to false as well, it also seems to have stabilized firefox, with "layers.offmainthreadcomposition.enabled" set to true FF was *very* unstable.
@Jaap
I had the same problem in firefox 40.0.3 if I set "layers. offmainthreadco mposition. enabled" to false in about:config, then I can also set gfx.xrender.enabled to false as well, it also seems to have stabilized firefox, with "layers. offmainthreadco mposition. enabled" set to true FF was *very* unstable.
Regards,
Steve.