I've found that the following query:

SELECT DISTINCT "Revision", COALESCE("Message", '') AS "LogMessage"
FROM (
SELECT "Revision", "StageID" FROM "tilemaps"
UNION SELECT "Revision", "StageID" FROM "entitymaps"
UNION SELECT "Revision", "StageID" FROM "log"
) AS "revisions"
NATURAL JOIN "log"
WHERE "StageID" = :stageId
ORDER BY "Revision" DESC

can systematically crash Firebird. I've tried running the server through a debugger on my x64 Linux system and it seems to me like a null pointer dereferencing. Here follows the backtrace:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff3b60700 (LWP 12346)]
MAKE_desc (statement=0x7ffff41661e8, desc=0x8, node=0x0, null_replacement=0x0)
    at ../src/dsql/make.cpp:409
409 if (node->nod_desc.dsc_dtype && !null_replacement)
(gdb) bt
#0 MAKE_desc (statement=0x7ffff41661e8, desc=0x8, node=0x0,
    null_replacement=0x0) at ../src/dsql/make.cpp:409
#1 0x000000000067cad2 in MAKE_desc_from_list (statement=0x7ffff41661e8,
    desc=0x7ffff3b5c130, node=0x7ffff40cdf40, null_replacement=0x0,
    expression_name=0x777fbb "COALESCE") at ../src/dsql/make.cpp:1494
#2 0x000000000067d76d in MAKE_desc (statement=0x7ffff41661e8,
    desc=0x7ffff40cd8a8, node=0x7ffff40cd800, null_replacement=0x0)
    at ../src/dsql/make.cpp:614
#3 0x000000000067cdfc in MAKE_desc (statement=0x7ffff41661e8,
    desc=0x7ffff40cd8a8, node=0x7ffff40cd770, null_replacement=0x0)
    at ../src/dsql/make.cpp:1178
#4 0x000000000069cfd7 in set_parameter_type (statement=0x7ffff41661e8,
    in_node=0x7ffff40cd8a0, node=0x7ffff40cd770, force_varchar=false)
    at ../src/dsql/pass1.cpp:10470
#5 0x00000000006a3be7 in PASS1_node (statement=0x7ffff41661e8,
    input=<optimized out>) at ../src/dsql/pass1.cpp:1155
#6 0x00000000006ac411 in pass1_node_psql (input=<optimized out>,
    statement=0x7ffff41661e8, psql=<optimized out>) at ../src/dsql/pass1.cpp:7333
#7 pass1_rse_impl (statement=0x7ffff41661e8, input=0x7ffff40f6010,
    order=0x7ffff40f5e28, rows=<optimized out>, update_lock=0x0,
    flags=<optimized out>) at ../src/dsql/pass1.cpp:8085
#8 0x00000000006ad00e in pass1_rse (flags=0, update_lock=0x0,
    rows=<optimized out>, order=<optimized out>, input=<optimized out>,
    statement=0x7ffff41661e8) at ../src/dsql/pass1.cpp:7959
#9 pass1_rse_impl (statement=0x7ffff41661e8, input=0x7ffff40f5eb0, order=0x0,
    rows=0x0, update_lock=0x0, flags=<optimized out>)
    at ../src/dsql/pass1.cpp:8011
#10 0x00000000006afb5c in pass1_rse (flags=0, update_lock=<optimized out>,
    rows=0x0, order=0x0, input=<optimized out>, statement=0x7ffff41661e8)
    at ../src/dsql/pass1.cpp:7959
#11 PASS1_rse (statement=0x7ffff41661e8, input=<optimized out>,
    update_lock=<optimized out>) at ../src/dsql/pass1.cpp:1232
#12 0x00000000006b6b0d in PASS1_statement (statement=0x7ffff41661e8,
    input=0x7ffff40f5dd8) at ../src/dsql/pass1.cpp:1867
#13 0x0000000000666909 in prepare (tdbb=0x7ffff3b5ec90,
    database=<optimized out>, transaction=0x7ffff416cd18, string_length=24024,
    string=<optimized out>, client_dialect=3, parser_version=2)
    at ../src/dsql/dsql.cpp:2625
#14 0x000000000066ae7c in DSQL_prepare (tdbb=0x7ffff3b5ec90,
    transaction=0x7ffff416cd18, req_handle=0x7ffff7feb7a0, length=302,
    string=0x7ffff7fc9188 "SELECT DISTINCT \"Revision\", COALESCE(\"Message\", '')---Type <return> to continue, or q <return> to quit---
 AS \"LogMessage\"\nFROM (\n\tSELECT \"Revision\", \"StageID\" FROM \"tilemaps\"\n\tUNION SELECT \"Revision\", \"StageID\" FROM \"entitymaps\"\n\tUNION SELECT \"Revision\","..., dialect=3, item_length=26,
    items=0x7ffff3b5f3d8 "~\025\004\a\t\v\f\r\016\020\021\022\023\b\005\a\t\v\f\r\016\020\021\022\023\b\265\363\377\177", buffer_length=32768,
    buffer=0x7ffff0a5fc08 "~\004") at ../src/dsql/dsql.cpp:726
#15 0x0000000000573d41 in jrd8_prepare (user_status=0x7ffff3b5f7f0,
    tra_handle=0x7ffff3b5eee0, stmt_handle=0x7ffff7feb7a0, length=302,
    string=0x7ffff7fc9188 "SELECT DISTINCT \"Revision\", COALESCE(\"Message\", '') AS \"LogMessage\"\nFROM (\n\tSELECT \"Revision\", \"StageID\" FROM \"tilemaps\"\n\tUNION SELECT \"Revision\", \"StageID\" FROM \"entitymaps\"\n\tUNION SELECT \"Revision\","..., dialect=32, item_length=26,
    items=0x7ffff3b5f3d8 "~\025\004\a\t\v\f\r\016\020\021\022\023\b\005\a\t\v\f\r\016\020\021\022\023\b\265\363\377\177", buffer_length=32768,
    buffer=0x7ffff0a5fc08 "~\004") at ../src/jrd/jrd.cpp:3892
#16 0x000000000043b7a6 in isc_dsql_prepare_m (user_status=<optimized out>,
    tra_handle=<optimized out>, stmt_handle=0x7ffff7feba00, length=302,
    string=0x7ffff7fc9188 "SELECT DISTINCT \"Revision\", COALESCE(\"Message\", '') AS \"LogMessage\"\nFROM (\n\tSELECT \"Revision\", \"StageID\" FROM \"tilemaps\"\n\tUNION SELECT \"Revision\", \"StageID\" FROM \"entitymaps\"\n\tUNION SELECT \"Revision\","..., dialect=32, item_length=26,
    items=0x7ffff3b5f3d8 "~\025\004\a\t\v\f\r\016\020\021\022\023\b\005\a\t\v\f\r\016\020\021\022\023\b\265\363\377\177", buffer_length=32768,
    buffer=0x7ffff0a5fc08 "~\004") at ../src/jrd/why.cpp:3629
#17 0x000000000041b702 in rem_port::prepare_statement (this=0x7ffff7fec578,
    prepareL=0x7ffff7feaa58, sendL=0x7ffff7fea388)
    at ../src/remote/server.cpp:3212
#18 0x0000000000420e6e in process_packet (port=0x7ffff7fec578,
    sendL=0x7ffff7fea388, receive=0x7ffff7fea798, result=0x7ffff3b5fe10)
    at ../src/remote/server.cpp:3516
#19 0x0000000000422a06 in loopThread () at ../src/remote/server.cpp:5212
#20 0x0000000000436706 in run (this=<synthetic pointer>)
    at ../src/jrd/ThreadStart.cpp:128
#21 (anonymous namespace)::threadStart (arg=0x7ffff7feac70)
    at ../src/jrd/ThreadStart.cpp:139
#22 0x00007ffff66e5df0 in start_thread () from /lib/libpthread.so.0
#23 0x00007ffff642b39d in clone () from /lib/libc.so.6
#24 0x0000000000000000 in ?? ()

I can give the statements used to create the affected tables if needed.