While updating Mozilla Firefox rcudt55.dll came under scrutiny.

See: http://systemexplorer.net/file-database/file/icudt55-dll
Checking with the cloud-driven aditional security program Crystal Security (running as a portable) C:\ProgramFiles(x86)\Mozilla Firefox\updated\rcudt55.dll had to be allowed to be downloaded, all other parts of the download were given the green light.
Seems this dll is benign and no threat. Was this because it is a minimal version?
It should be given the all green: https://www.reasoncoresecurity.com/icudt55.dll-5293f8ef23d8a0e43e7f3bddefcedcb13235726f.aspx

But as our good forum friend, Para_Noid, always says, “Only that is found to be secure what we have checked ourselves to be so”.
And polonus thinks he is right there.

polonus