The IE Tab add on is an “extension”.
- Extensions do not get picked up across mozilla-based browsers. In general, extensions modify the browser’s UI, so it would make sense that they only affect the browser they were specifically installed for.
- Plugins affect types of media content that can be played back in the browser. When they’re installed, they usually make a change to the system registry so that they identify themselves as the appropriate player for the type of media they target.
So for instance, Flash, VLC, Quicktime, and Silverlight are all plugins. Because they identify themselves in the system registry, all browsers will pick up the plugin once installed (as long as the other browsers share the same plugin architecture - IE and IE-based browsers look for ActiveX plugins, Firefox and Gecko-based browsers look for NPAPI plugins).
There is at least one exception though - the Windows Media Player plugin for Firefox is a little weird. It doesn’t properly register itself in the system, so you have to manually install it per browser.
One more note: even though most of the time a plugin (like VLC or Flash) installed via Firefox will get picked up by Gecko-based browsers like Kylo - you can override with a different version by manually installing the plugin to the \Plugins folder under the browser’s install location (ie. C:\Program Files (x86)\Hillcrest Labs\Kylo\Plugins).
Here’s a write up on the difference between plugins and extensions: