I believe to have found at problem in FbConnectionPoolManager. In my case the problem shows on a website where there is a spike in user activity in the morning, the user activity results in about 75-100 attachments to the database. However after the spike, the connections/attachments to the database remains. The Connection lifetime is set to 15 in my connectionstring.
After digging around in the sourcecode I believe I have found the problem in FbConnectionPoolManager, where a Queue<Item> is used to hold available connections. The FIFO nature of the Queue results in every available connections to be checked out of the pool within 15 seconds if there is enough database activity. Thus keeping the 75-100 connection/attachment to the database open for a long period (usually 6-8 hours)
I believe using a Stack<Item> instead of Queue<Item> could solve the problem.