This problem is now solved. The problem was that "offmainthread" rendering requires that SKIA [1] graphics library must be compiled-in at compile time.
In firefox-45-esr SKIA appears to be disabled by default, and requires opt-in at compile time to enable it. "offmainthread" is enabled by default (but can be disabled by about:config entries). By using all defaults, you end up with incorrectly working Firefox binary. In firefox-52-esr SKIA appears to be enabled by default, and can be opt-out at compile time to disable it. "offmainthread" is hard-coded-enabled. By using all defaults, you end up with correctly working Firefox binary. My problem with firefox-52-esr builds was that I had these in my .mozconfig ac_add_options --disable-skia ac_add_options --disable-skia-gpu ac_add_options --disable-skia-pdf [1] https://en.wikipedia.org/wiki/Skia_Graphics_Engine -- Jari Ruusu 4096R/8132F189 12D6 4C3A DCDA 0AA4 27BD ACDF F073 3C80 8132 F189
