Justinmind is not available for Linux but there are plenty of alternatives that runs on Linux with similar functionality. The best Linux alternative is Pencil Project, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Justinmind and 14 are available for Linux so hopefully you can find a suitable replacement. Other interesting Linux alternatives to Justinmind are Figma (Freemium), quickMockup (Free, Open Source), mydraft.cc (Free, Open Source) and WireframeSketcher (Paid).
EaSynth ForeUI is an easy-to-use UI prototyping tool. You can use ForeUI to create skinnable prototype of software or website, and run the interactive simulation in your web browser. You can switch the UI theme of the prototype, thus the fidelity will be changed as well.