I cannot tell you why the Gunbroker website will make your Firefox hang in case the Java plugin has been enabled.
Basically, there are two lines of Java software plus Java browser plugin:
- OpenJDK Java. The corresponding browser plugin is IcedTea. - For legal reasons, Linux distributions like Ubuntu und Mint are allowed to offer them in their repositories for downloading and installing.
- Oracle Java and Oracle Java browser plugin. - For legal reasons, Linux distributions like Ubuntu und Mint are NOT allowed to offer them in their repositories for downloading and installing.
Each end user is allowed to download them and install them. Yet, doing so is a manual procedure. There is a PPA repository, however, which performs the manual steps for you.
About Java versions:
No matter whether you use OpenJDK or Oracle Java, the Java version 1.6.x has been discontinued and is known to hold severe security vulnerabilities. These are actively exploited by malicious websites.
Only recently two persons' computers whom I know were infected by the same trojan horse (ransomware) which had made its way onto the machines by exploiting a known hole in the Java plugin 1.6_27 and older.
True both were Windows machines. So the trojan horse would not have worked on Mint.
Nonetheless I will not have any Java 1.6.x on any machine, but replace it by Java 1.7_25.
But even Java 1.7_25 cannot be considered safe.
So as a rule I disable the Java browser plugins by default, no matter whether it is the Oracle Java plugin or the OpenJDK plugin named IcedTea.
Websites in the internet that require Java inside the browser are rare.
There are, however, quite a few commercial office programes which are used by companies in their internal network that run inside a web browser and that depend on Java.