Comment 6 for bug 808752

I let thunderbird run for some time, monitoring the number of open files, and it doesn't seem to leak file descriptors. But it opens 1fd per imap directory accessed, then release it after some time.

Just a supposition, if you have many imap folders with mail filters, thunderbird can quickly reach the limit of 1024 open files.
For example, locally if I only count files which are not mail folders, there are already 280 descriptors used, that leaves only ~740 free descriptors for mail directories. Is 'many cascading IMAP folders' of that order ?

Could you increase the hard limit of open files and see if it improves the situation ?

You can change the max number of open files by adding the following lines to /etc/security/limits.conf
@your_username hard nofile 4096
@your_username soft nofile 4096

and logout/in