Thunderbird 68.x segfault

In both 14.2 and Current, Thunderbird 68.x will segfault when gfx.downloadable_fonts.enabled=false. Original discussion here.

I downloaded the Mozilla pre-built.

No segfault. Curious. I don't know if that means a glitch with the build Slackware environment for Thunderbird.

Using the Slackware build script I compiled Thunderbird 68.2.1 without --disable-debug and --enable-strip. A gdb /usr/lib64/thunderbird/thunderbird "$@" backtrace:

Thread 1 "thunderbird" received signal SIGSEGV, Segmentation fault.
0x00007fffe686668b in ?? () from /usr/lib64/thunderbird/libxul.so
(gdb) bt
#0  0x00007fffe686668b in  () at /usr/lib64/thunderbird/libxul.so
#1  0x00007fffe71f8cc2 in  () at /usr/lib64/thunderbird/libxul.so
#2  0x00007fffe71f8a5b in  () at /usr/lib64/thunderbird/libxul.so
#3  0x00007fffe8a63f91 in  () at /usr/lib64/thunderbird/libxul.so
#4  0x00007fffe896c359 in  () at /usr/lib64/thunderbird/libxul.so
#5  0x00007fffe81dff09 in  () at /usr/lib64/thunderbird/libxul.so
#6  0x00007fffe5601e2e in  () at /usr/lib64/thunderbird/libxul.so
#7  0x00007fffe568bf91 in  () at /usr/lib64/thunderbird/libxul.so
#8  0x00007fffe565944a in  () at /usr/lib64/thunderbird/libxul.so
#9  0x00007fffe3ff7474 in  () at /usr/lib64/thunderbird/libxul.so
#10 0x00007fffe3ff6ed0 in  () at /usr/lib64/thunderbird/libxul.so
#11 0x00007fffe3ff95f9 in  () at /usr/lib64/thunderbird/libxul.so
#12 0x00007fffe4239b0c in  () at /usr/lib64/thunderbird/libxul.so
#13 0x00007fffe49a05fa in  () at /usr/lib64/thunderbird/libxul.so
#14 0x00007fffe69a0ca5 in  () at /usr/lib64/thunderbird/libxul.so
#15 0x00007fffe699a63e in  () at /usr/lib64/thunderbird/libxul.so
#16 0x00007fffe6990b87 in  () at /usr/lib64/thunderbird/libxul.so
#17 0x00007fffe69a0f75 in  () at /usr/lib64/thunderbird/libxul.so
#18 0x00007fffe69a1406 in  () at /usr/lib64/thunderbird/libxul.so
#19 0x00007fffe6b0619f in  () at /usr/lib64/thunderbird/libxul.so
#20 0x00007fffe69a0ca5 in  () at /usr/lib64/thunderbird/libxul.so
#21 0x00007fffe699a63e in  () at /usr/lib64/thunderbird/libxul.so
#22 0x00007fffe6990b87 in  () at /usr/lib64/thunderbird/libxul.so
#23 0x00007fffe69a0f75 in  () at /usr/lib64/thunderbird/libxul.so
#24 0x00007fffe69a1406 in  () at /usr/lib64/thunderbird/libxul.so
#25 0x00007fffe6cf48d0 in  () at /usr/lib64/thunderbird/libxul.so
#26 0x00007fffe478553d in  () at /usr/lib64/thunderbird/libxul.so
#27 0x00007fffe4bf8929 in  () at /usr/lib64/thunderbird/libxul.so
#28 0x00007fffe4be4dec in  () at /usr/lib64/thunderbird/libxul.so
#29 0x00007fffe4be56d3 in  () at /usr/lib64/thunderbird/libxul.so
#30 0x00007fffe4bde520 in  () at /usr/lib64/thunderbird/libxul.so
#31 0x00007fffe4bddf35 in  () at /usr/lib64/thunderbird/libxul.so
#32 0x00007fffe4bdf823 in  () at /usr/lib64/thunderbird/libxul.so
#33 0x00007fffe56999ba in  () at /usr/lib64/thunderbird/libxul.so
#34 0x00007fffe64bcc68 in  () at /usr/lib64/thunderbird/libxul.so
#35 0x00007fffe64bc987 in  () at /usr/lib64/thunderbird/libxul.so
#36 0x00007fffe64bdc4d in  () at /usr/lib64/thunderbird/libxul.so
#37 0x00007fffe39520a7 in  () at /usr/lib64/thunderbird/libxul.so
---Type <return> to continue, or q <return> to quit---
#38 0x00007fffe3951cb2 in  () at /usr/lib64/thunderbird/libxul.so
#39 0x00007fffe3950901 in  () at /usr/lib64/thunderbird/libxul.so
#40 0x00007fffe3951683 in  () at /usr/lib64/thunderbird/libxul.so
#41 0x00007fffe3951b9a in  () at /usr/lib64/thunderbird/libxul.so
#42 0x00007fffe2e86eba in  () at /usr/lib64/thunderbird/libxul.so
#43 0x00007fffe3dfec70 in  () at /usr/lib64/thunderbird/libxul.so
#44 0x00007fffe3e00efd in  () at /usr/lib64/thunderbird/libxul.so
#45 0x00007fffe3df924c in  () at /usr/lib64/thunderbird/libxul.so
#46 0x00007fffe3df8cf1 in  () at /usr/lib64/thunderbird/libxul.so
#47 0x00007fffe3dd41af in  () at /usr/lib64/thunderbird/libxul.so
#48 0x00007fffe3de65bc in  () at /usr/lib64/thunderbird/libxul.so
#49 0x00007fffe3df23a4 in  () at /usr/lib64/thunderbird/libxul.so
#50 0x00007fffe4be4dec in  () at /usr/lib64/thunderbird/libxul.so
#51 0x00007fffe4be56f4 in  () at /usr/lib64/thunderbird/libxul.so
#52 0x00007fffe4bde520 in  () at /usr/lib64/thunderbird/libxul.so
#53 0x00007fffe4bddf35 in  () at /usr/lib64/thunderbird/libxul.so
#54 0x00007fffe4bdf823 in  () at /usr/lib64/thunderbird/libxul.so
#55 0x00007fffe4be128d in  () at /usr/lib64/thunderbird/libxul.so
#56 0x00007fffe3ff6420 in  () at /usr/lib64/thunderbird/libxul.so
#57 0x00007fffe4be92fb in  () at /usr/lib64/thunderbird/libxul.so
#58 0x00007fffe4bc0055 in  () at /usr/lib64/thunderbird/libxul.so
#59 0x00007fffe2de8536 in  () at /usr/lib64/thunderbird/libxul.so
#60 0x00007fffe2dea246 in  () at /usr/lib64/thunderbird/libxul.so
#61 0x00007fffe33b3bf7 in  () at /usr/lib64/thunderbird/libxul.so
#62 0x00007fffe3348df8 in  () at /usr/lib64/thunderbird/libxul.so
#63 0x00007fffe54b0076 in  () at /usr/lib64/thunderbird/libxul.so
#64 0x00007fffe679453e in  () at /usr/lib64/thunderbird/libxul.so
#65 0x00007fffe68656c0 in  () at /usr/lib64/thunderbird/libxul.so
#66 0x00007fffe6865e39 in  () at /usr/lib64/thunderbird/libxul.so
#67 0x00007fffe6866305 in  () at /usr/lib64/thunderbird/libxul.so
#68 0x000055555555a8bc in  ()
#69 0x00007ffff6d717d0 in __libc_start_main () at /lib64/libc.so.6
#70 0x000055555555a3b9 in _start ()

Running with valgrind -v /usr/lib64/thunderbird/thunderbird "$@":

Numerous Address 0x20247070 is in a rw- anonymous segment like errors:

==30264== Invalid free() / delete / delete[] / realloc()
==30264==    at 0x4C2F14B: free (vg_replace_malloc.c:540)
==30264==    by 0x1638370F: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x1638B657: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x16371A74: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x160157F5: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x160154C0: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x161554D0: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x162B63C0: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x12D82FE3: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x12D66475: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x14F2ADA5: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==    by 0x1239889C: ??? (in /usr/lib64/thunderbird/libxul.so)
==30264==  Address 0x20247070 is in a rw- anonymous segment
==30264== 
==30264== ERROR SUMMARY: 112933 errors from 513 contexts (suppressed: 0 from 0)
Segmentation fault

Seems related to libxul.so or glibc?

I'm not a code guru. Any help or hints are appreciated to find the actual cause.

if you are not a code guru you will hardly find the real reason. Most probably a pointer to a variable was passed to a function which led to the call to free (and vg_replace_malloc), but that pointer was miscalculated/already freed, the memory was already reused .... or just you mixed some incompatible libraries. Hard to say anything.
Based on the previous threads you need to check the code related to [handling] downloadable fonts.
You might want to check that part with valgrind.

Based on the Mozilla pre-built not having the error, my first guess is the way Thunderbird is compiled in Slackware.

yes, that may mean incompatible libraries. You may try to replace libxul.so (or other libraries). I mean from the pre-built package. And if you found something usable you need to find again the reason.

You may try to replace libxul.so.

That fails because of a link to a wayland lib.