Hi there -- Thanks for letting us know. What version of vCenter Server / vSphere client are you using? The VMRC plugin was deprecated in newer versions of vSphere (5.5 update 2b and later) and the native desktop VMRC can be found at www.vmware.com/go/download-vmrc. (VMRC 10.0.2 was just released yesterday.)
Does the Open with VMRC link work with a different browser and Fedora 26?
Running the installer bundle provided by the response works for me. Didn't even need to restart Firefox 57; recognized vmrc immediately once installed. ArchLinux x64.
Registering custom protocol handler in firefox helped me to get around this issue:
Basically you have to register network.protocol-handler.expose.vmrc;false after which firefox will prompt which program to use etc