First of all, please ensure that you are using the latest version of SimpleViewer (v2.3.2) to ensure that any bugs which were present in previous versions but which have since been fixed are not contributing to your problem.
Instructions for downloading the latest version and for upgrading existing galleries can be found here.
Next, try clearing your Firefox browser's cache to ensure that the browser is fetching and using the most recent versions of your gallery files: https://support.mozilla.org/en-US/kb/ho … efox-cache
Also, check that your Firefox browser has the latest version of Adobe Flash Player installed by visiting this web page: https://helpx.adobe.com/flash-player.html
If you continue to experience difficulties, please post the URL to your gallery so that I can take a look at the problem for myself and hopefully help further.
SimpleViewer Support Team