Extensions are options you install into Firefox, They can be removed at will. Plugins are from programs that Firefox finds already on your system. The only way to remove a plugin is to remove its program from your system.

https://support.mozilla.org/en-US/kb/update-firefox-latest-version 46.0.1

Fuzzy31 said

ADD-Ons have the choise: de-activate or remove. Why plugins don´t have this option?

Extensions and complete Themes are installed in Firefox while Plugins are installed on system whether on Windows, Mac OSX or Linux.

It the Location (address) bar type about:plugins ..with a :

This will then show you where the Plugins that Firefox is making use of is located on system.

https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix-problems#w_manually-uninstalling-a-plugin

Thanx, the problem is solved - with "about:plugins". I removed all old / unneeded plugins. Only the JAVA Development Plugin is still there, but I don´t remove, because it belongs to JAVA 9 EA and I won´t remove JAVA 9.

Now all is O.K., Thanx once more and have a nice weekend / time. With b est regards, Fuzzy31.

Oracle is planning to depreciate the Java Plugin in Java 9. https://blogs.oracle.com/java-platform-group/entry/moving_to_a_plugin_free