Firefox ESR (Extended Support Release) version 52, 32-bit, is a version of Firefox that supports Java applets.
You can install it on your machine (as described below) and then use it as your standard version of Firefox or you can keep the current personal version of Firefox (currently version 55) and run one or the other as you like.
You can't have both running at the same time, without some effort (and some risk).
If you want further advice on how to install Firefox ESR, try this.

To install Firefox ESR

• Go to the Firefox ESR download page
   
• The page will advise you that Firefox ESR is for organizations, while individuals who want the latest features should use normal Firefox.
  Ignore this. You do not want the latest features. Amongst the latest features is removal of support for Java.
   
• Click Download Mozilla Firefox ESR
   
• Note that there are five download columns. If you are using Firefox in Windows, you'll choose from the first column (labeled "Windows"). Do NOT choose from the second column (labeled "Windows 64-bit"), even if you have a 64-bit computer. Only the first version (32-bit) supports Java. It will work fine on a 64-bit computer.
   
• If you are using Firefox in macOS, choose that column. If in Linux, you should probably choose the first Linux column (not Linux 64-bit). I don't know if these will work.
   
• Scroll down to your favorite language, and click download in the appropriate column.
   
• Save the file and execute it
   
• During the installation you'll be asked for the directory in which to install Firefox ESR. You can either overwrite your old Firefox or create a new directory with a different name to preserve your old Firefox.
   
• Once Firefox ESR is installed run it. You can tell you've got the right version by clicking Help and then About Firefox. It should say "Firefox ESR... (32 bit)"
   
• When you access a webpage that runs a Java applet, it will ask you whether you want to run this application. Yes, you do.

(If you have any other advice that would work better than the above, please let me know)