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Plugins versus Extensions

An attempt to cut through the confusion created by Mozilla developers

Copyright © 2017, 2021 by David E. Ross

If it ain't broke, don't fix it.

Change merely for the sake of change is not only stupid. It is wasteful and can also be harmful.

An important feature of several different browsers is the ability to add extensions to modify or add capabilities. Beyond extensions, plugins can add capabilities not only to browsers but also to other applications.

Two unfortunate decisions were made by the developers of Mozilla-based applications with respect to both plugins and extensions:

To make sense of all this, users need to understand the differences between plugins and extensions. The differences are indeed major.


A plugin is a software application that can be used by another application.

On my PC, the same plugin (doPDf) to "print" to a PDF file is available to my browser (SeaMonkey), my E-mail client (Thunderbird), Word, Excel, GIMP (GNU Image Manipulation Program), and others. It is even available to Adobe Reader, where I use it to strip away locks that prevent modifications. I installed doPDF only once to make it available to all. However, it actually installed as a plugin to my PC's print server. I could have enabled its plugin for Word and Excel, but I disabled that.

For SeaMonkey, I have enabled one plugins, an old version of Flash — a version prior to the ones that self-destructed — needed by one Web-based game I sometimes play. I have no plugins enabled for Thunderbird.

Note that, while the Mozilla browser Firefox now blocks plugins, plugins will still be available for non-Mozilla applications.


An extension can be perceived as a software fragment for a single application.

On my PC, I have 26 extensions installed for SeaMonkey:

Also on my PC, I have 12 extensions installed for Thunderbird. These can be characterized similarly to those for SeaMonkey. Indeed, four of them are the same as those I installed for SeaMonkey. When new versions of those four are released, I must install them twice, once each for SeaMonkey and Thunderbird.

The Future for Mozilla

Plugins are already blocked in Firefox, and this change is expected to propagate soon to SeaMonkey and Thunderbird. Additionally, the interface for extensions will block those that have the .xpi file-extension. Following a trend to make Mozilla-based applications more like Google's Chrome, Firefox will only accept Webextensions; this change too is expected to propagate soon to SeaMonkey and Thunderbird. I see two major problems with this change to the extension interface:

I continued to use Eudora Lite as my E-mail client long after Qualcomm stopped maintaining it. I changed to Thunderbird only after Eudora Lite would not interface with the E-mail server of a new ISP to which I was forced to change; soon thereafter, Eudora Lite would install in a newer version of Windows. I plan to use the last versions of not Chrome-like SeaMonkey and Thunderbird as long as they remain functional and can be installed.

7 July 2017
Updated 21 December 2021

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