Due to new security enhancements in Firefox version 52, there are some important things to consider when running Oracle Forms in Firefox. As of March 7, 2017, Mozilla has released the newest release of Firefox, version 52. Similar to what Google Chrome did about a year or so ago, Mozilla has now disabled the NPAPI plug-in which is required to run anything with Java such as Oracle Forms without using Java Web Start. What will happen is if Firefox is updated to version 52 and you try to access Forms, you will see a blank screen which will result in the Forms application to not start at all.
Fortunately, Mozilla has realized that businesses still need to use services such as Java during this transition, so Mozilla has released the Firefox “Extended Support Release” for version 52. This is a version of Firefox which is used by companies and organizations which need support for running Java with the NPAPI plug-in especially for mass deployments. The current ESR version of 52 will continue to allow the use of the NPAPI plug-in for Java through May 2018. The download link (to download the 32-bit Firefox 52 ESR for Windows 32 and 64-bit machines) may be found here:
After this is installed on your PC, you will be able to run Forms using Firefox again without the need of Java Web Start. This is important for anyone who is still running Forms 11gR2 (or older) which has no support for Java Web Start. However, if you are running Forms 12c, you may use Java Web Start which allows you to run Java (required for running Oracle Forms) without the NPAPI plug-in. Java Web Start allows you to run Oracle Forms 12c from any web browser (Internet Explorer, Firefox, Chrome, Edge, etc.). If you decide to use Java Web Start, the Firefox ESR is not necessary.
Sources: https://support.mozilla.org/t5/Problems-with-add-ons-plugins-or/Why-do-Java-Silverlight-Adobe-Acrobat-and-other-plugins-no/ta-p/31069 and https://www.fxsitecompat.com/en-CA/docs/2016/plug-in-support-has-been-dropped-other-than-flash/