Windows 10 64bit
ThunderbirdPortable 52.5.0 32bit
If mailto links are assigned to be opened with ThunderbirdPortable, it seems to truncate the command line arguments it sends to thunderbird.exe.
I've tested the same long mailto link, almost 1000 chars long (the raw url encoded version) with predefined body etc, with the same profile in both portable thunderbird and a normal thunderbird installation. In the normal installation (same version 52.5.0 32bit) it works just fine. I've tested the mailto link in Firefox 57, Edge and IE11, all with the same result.