Thanks to its multi-generational architecture Firebird does not block readers. So it’s natural that this problem may seem strange to us. But each architecture has its pros and cons. This same multi-generational architecture, if not correctly used, can be a problem. It’s the case of long running transactions or "stuck transactions".
Such transactions cause record versions to accumulate in the database. (These versions can also be called generations. It’s multi-generational architecture, remember?) The more versions accumulate more work Firebird has to do to find the correct version of each record. If stuck for long enough Firebird server can get slow to the point of looking like it’s dead. How much time depends of the database load and the server’s hardware. Could be a month, could be an hour.
I’ve seen it happen many times. In some of those occasions I saw managers argue for hours that MS SQL Server or Oracle don’t have such problem. The fact is that they have other architectural details that you must take into account in your application. For example, some databases block readers. Simply switching databases, as some of those managers suggested, is not a solution.
Although having StackOverflow powered by Firebird would not be a bad idea. ;)