First of all regarding yesterday file. Unfortunately "Non-text portions of thAT message have been removed". So please upload that file to any public file-sharing you better like (except yandex & mail.ru). Sorry for inconveniences - that's not me who invited yahoo interface...

> But you should write me what and where must be configured to obtain some error dumps when it falls down.

Install appropriate debug info (in our builds it's always placed as a pair for binary package). After it adding BugcheckAbort=1 to firebird.conf should be enough. And get ready that this may be a long process - may be I will have to send you some special builds with additional debugging.

> Because firebird didn't restarted itself since yesterday afternoon then

I'll have to restart it manually to enable legacy authentication.

That's bad solution but that's your solution. That backward compatibility feature was kept in firebird for such cases.

>Together with this change I'm thinking about moving firebird temp dir to

tmpfs because I've read on some IBSurgeon pages that it might improve perfomance.

>(In my Debian Stretch directory /run seems suitable for this.)
>Do you think it's really worth it?

That highly depends upon RAM usage on your box. If you have enough RAM - that's useful, if not - may cause performance degradation.

Also please provide info about your kernel, libc and libstdc++ versions.