We are using Thunderbird for a couple of years. About week ago we started havin problems with the pdf files... yes.... and only the pdf files. When i try to download/open a pdf file from the attachment, i have to wait about 20 second for 100 kb pdf file, and this problem is not from the size of the attachment because i have tried with a 50 mb pdf file with the same result. The problem is that we are using proxy for network filtering and with the help of the Wireshark i have found, that every time you try to open or download a pdf file in thunderbird, a communication is started with this IP 172.217.19.110 . It seems that you have to wait until it gets a time out, and after that you can open/download your pdf file.

As you can see in the picture, if i disable the proxy filter for my ip everything is working like it should.