I tried to reproduce the issue on my PC but the True Key extension got installed successfully. I tested this on Windows 10 PC with FF 57.0 32 bit. Please go to the list of add-ons and check if True Key is already available, it is possible that it is disabled.
If True Key extension isnt available, try downloading again from https://www.truekey.com/onboard/ and check if it gets installed.
True key does not see on the list in 'about:addons' and i tried my another laptop but same issue. (both PC windows 10 64bit build:1709)
it's maybe affected by another app like antivirus or something internet issue.
anyone could you give me latest true key addon file? i'll try manually install addon
Thank you for your help.