Hallo!
Nach eurer Beschreibung passt der Bug zu etwas das wir im Bugtracker von Mozilla gefunden haben. Leider sind die Informationen hier auf Englisch. Es scheint jedoch so zu sein, dass Firefox in Kombination mit Java gerne komplett abstürzt.
Folgendes Findet man dazu auf der Seite von Java.
Zitat:
Firefox 42 and later versions crash when trying to run the Java plug-in

NOTE: The workaround applies to Firefox 42 and later versions, and requires Windows Java Version 8 Update 66 Build 18 or later versions. » See JDK-8133523 for related information.

Workaround
A few options are listed. The simplest method is noted first and recommended. Other available options are not recommended for non-technical users.

Option 1 (Simplest method)
Add property name dom.ipc.plugins.java.enabled in Firefox preferences
Launch Firefox
Type about:config in the address bar
You will be presented with a Firefox warning about the configuration settings. After acknowledging the Firefox alert, you should see a listing of the configuration preferences.
Right-click anywhere inside the displayed list of preferences. Select New then select Boolean.
Add property name as dom.ipc.plugins.java.enabled
Add Preference name as dom.ipc.plugins.java.enabled.
A window will be displayed, select false. Then select OK.
Option 2
Create "user.js" file with preference entry
Instructions are based on Mozilla references listed below.

Use a text editor to create a "user.js" file in your Firefox profiles folder
Add entry:
user_pref("dom.ipc.plugins.java.enabled", false);
Option 3
Update "pref.js" file with preference entry
Locate the "pref.js" file in the Mozilla profiles folder
Find and update preference
user_pref("dom.ipc.plugins.java.enabled", false);
Mozilla references
» http://kb.mozillazine.org/User.js_file
» http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder


Für jeder der von java genannten Lösungen benötigt ihr jedoch zwingend die neueste Version von Java.
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"A gem cannot be polished without friction, nor a man without trials." - Seneca