In case I need to switch to a new PC anytime:
1. Update Java to the latest version (from java.com)
2. Open your control panel (for W10: right-click the start button, choose control panel)
3. In the search box in the top right of the window, type: java
4. Click the java icon
5. Switch to the "Security" tab
6. Towards the bottom, under "Exception Site List", click the "Edit Site List..." button.
7. Click the "Add" button.
8. Click the line under "Location" (where there's an example https url)
9. Type:
https://fumbbl.com
10. Press Enter
11. Press "OK"
12. Press "OK" again
13. Done. You can now launch the client.