It's a minor gdb error, but still:

$ gdb -p $firefox_pid
...
$ info threads
...
  53   Thread 0x7fffdd0ff640 (LWP 2653) "DOM Worker"     
__futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393,
expected=0, futex_word=0x7fffddc1b84c) at futex-internal.c:57
  54   Thread 0x7fffe52ff640 (LWP 2654) "StyleThread#0"  
__futex_abstimed_wait_common64 (private=<error reading variable: Dwarf Error:
Cannot find DIE at 0x4cd61173 referenced from DIE at 0x4c8e5b54 [in module
/home/marxin/.cache/debuginfod_client/0334765fb8f9ee4d799df184e904f19a128e9a09/debuginfo]>,
cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7fffdd69d1bc) at
futex-internal.c:57
  55   Thread 0x7fffe52be640 (LWP 2655) "StyleThread#1"  
__futex_abstimed_wait_common64 (private=<error reading variable: Dwarf Error:
Cannot find DIE at 0x4cd61173 referenced from DIE at 0x4c8e5b54 [in module
/home/marxin/.cache/debuginfod_client/0334765fb8f9ee4d799df184e904f19a128e9a09/debuginfo]>,
cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7fffdd69d36c) at
futex-internal.c:57
  56   Thread 0x7fffe04fd640 (LWP 2656) "StyleThread#2"  
__futex_abstimed_wait_common64 (private=<error reading variable: Dwarf Error:
Cannot find DIE at 0x4cd61173 referenced from DIE at 0x4c8e5b54 [in module
/home/marxin/.cache/debuginfod_client/0334765fb8f9ee4d799df184e904f19a128e9a09/debuginfo]>,
cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7fffdd69d90c) at
futex-internal.c:57
  57   Thread 0x7fffe527d640 (LWP 2657) "StyleThread#3"  
__futex_abstimed_wait_common64 (private=<error reading variable: Dwarf Error:
Cannot find DIE at 0x4cd61173 referenced from DIE at 0x4c8e5b54 [in module
/home/marxin/.cache/debuginfod_client/0334765fb8f9ee4d799df184e904f19a128e9a09/debuginfo]>,
cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7fffdd69dabc) at
futex-internal.c:57
...