Follow the instructions on this page to download updated .js files and replace them in the specified location in your RoboHelp installation.
We recommend that you take a backup of the existing .js files before replacing them with the files you download from this page.
This issue has been fixed in Update 9 of RoboHelp Classic (2019 release).
After performing these steps, the local Web Help output loads properly in Firefox. However, some features might still have limited availability. To fix this issue, perform the following additional steps:
If you do not have any business reason to access the local file system, then it is recommended to turn the security setting back to true.