Is that the entire output of redirecting firefox? Firefox 2>&1 | tee firefox.log
Here I get,
DIVE is disabled - Panorama's shadow-buffer is enabled
Assertion failed: fcCacheChains[i] == NULL, file fccache.c, line 533
Killed by SIGABRT
pid=0x007d ppid=0x0021 tid=0x0001 slot=0x0102 pri=0x0200 mc=0x0001 ps=0x0017
Note that the sigabrt comes from fontconfig.
Snap should automatically enable DIVE if it passes an sanity check, including using 32 bit colour.