I'm still seeing massive CPU overload with Firefox 68.0.1 and e.g. electron-6.

Attaching gdb to a running /usr/lib/firefox/firefox shows that, as soon as electron is started, the firefox process gets a *massive* amount of SIGSYS signals. Apparently all due to looking for font files. And each firefox process does gets these signals which seems to cause the overload, we're talking about ca 12000 signals on my system, probably due to the great number of font files I have installed here. Here are some excerpts from the debugging session:

Thread 1 "Web Content" received signal SIGSYS, Bad system call.
0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8cff489aa0 "/etc/fonts/fonts.conf", buf=0x7fffe2acfb70) at ../sysdeps/unix/sysv/linux/wordsize-64/xstat.c:35
Thread 1 "Web Content" received signal SIGSYS, Bad system call.
0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8cff678d00 "/etc/fonts/conf.d", buf=0x7fffe2acfb70) at ../sysdeps/unix/sysv/linux/wordsize-64/xstat.c:35

etc. Other locations/syscalls:

0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8d056b6a30 "/etc/fonts/conf.d/10-antialias.conf", buf=0x7fffe2acfb70)
    at ../sysdeps/unix/sysv/linux/wordsize-64/xstat.c:35
0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8d056b6b20 "/etc/fonts/conf.d/10-hinting-slight.conf", buf=0x7fffe2acfb70)
    at ../sysdeps/unix/sysv/linux/wordsize-64/xstat.c:35
0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8d056b6b50 "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", buf=0x7fffe2acfb70)
0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8d056b6b80 "/etc/fonts/conf.d/11-lcdfilter-default.conf", buf=0x7fffe2acfb70)
0x00007f8d210fe247 in __access (file=0x7f8cff695d40 "/etc/fonts/conf.d/20-unhint-small-dejavu-lgc-sans.conf", type=4) at ../sysdeps/unix/sysv/linux/access.c:27
0x00007f8d210fdd19 in __libc_open64 (file=0x7f8cff695ec0 "/etc/fonts/conf.d/20-unhint-small-dejavu-lgc-serif.conf", oflag=524288) at ../sysdeps/unix/sysv/linux/open64.c:47
0x00007f8d210fdd19 in __libc_open64 (file=0x7f8cff443c40 "/var/cache/fontconfig//3830d5c3ddfd5cd38a049b759396e72e-le64.cache-7", oflag=524290)
0x00007f8d210fd775 in __GI___xstat (vers=<optimized out>, name=0x7f8cff69bc40 "/usr/share/texmf/fonts/opentype/public/lm/lmmonocaps10-regular.otf", buf=0x7fffe2acf9b0)
0x00007f8d21b95dae in __libc_open64 (file=0x7f8cff6f89c0 "/usr/share/texmf/fonts/opentype/public/lm/lmroman9-italic.otf", oflag=0) at ../sysdeps/unix/sysv/linux/open64.c:47
0x00007f8d21b95dae in __libc_open64 (file=0x7f8cff658d80 "/usr/share/fonts/cMap/.resource/Identity-UTF16-H", oflag=0) at ../sysdeps/unix/sysv/linux/open64.c:47
0x00007f8d210fe247 in __access (file=0x7f8d05880060 "/var", type=0) at ../sysdeps/unix/sysv/linux/access.c:27
0x00007f8d210fe247 in __access (file=0x7f8d05880068 "", type=0) at ../sysdeps/unix/sysv/linux/access.c:27


etcpp. There are literally thausands of "received signal SIGSYS, Bad system call." messages. The last line pasted above looks interesting also, note that the path is empty, like some script going bonkers...

So it's probably something in recent libchromiumcontent versions that causes FF to process/search/etc all font files on the system, and for some reason each of those results in a "signal SIGSYS, Bad system call".