Silverlight China

A plain-English reference for the Silverlight era — and what to run instead

Can an Old Firefox Version Still Run Silverlight?

The short answer

Technically, yes: Firefox versions from before the plugin cutoff can still load the Silverlight plugin, because they still contain the NPAPI plugin machinery that Silverlight depends on. Practically, it is a dead end. You would be running an out-of-date browser and an out-of-support runtime just to reach one application, and nothing in that stack receives fixes anymore.

This page explains where the cutoff sits and what the trade-off really is, so you can decide whether the workaround is worth even a temporary look.

Where the cutoff is: Firefox 52

Mozilla announced in October 2015 that Firefox would end support for NPAPI plugins, the aging plugin interface that Silverlight, Java, and similar plugins all used. The change landed in Firefox 52, released in March 2017, whose release notes state plainly that NPAPI support was removed for plugins other than Flash — Silverlight, Java, and Acrobat among them.

So the dividing line looks like this:

  • Firefox 51 and earlier — the browser can still load NPAPI plugins, including Silverlight, if the plugin is installed on the machine.
  • Firefox 52 and later — the Silverlight plugin is invisible to the browser. It can be installed on the system, but Firefox will never load it, which is why pages just show a blank region or an endless "install Silverlight" prompt. We cover that broader picture in why Silverlight won't run in modern browsers.

Firefox's extended support release line of that era retained plugin support somewhat longer than the mainstream release channel, but no currently maintained Firefox build loads NPAPI plugins. Whatever copy of Firefox still runs Silverlight today is, by definition, a build that stopped receiving updates years ago.

What you are actually signing up for

Running a pre-52 Firefox means accepting three layers of abandoned software at once:

  • The browser itself. An old Firefox build no longer receives security fixes, and browsers are among the most heavily attacked software on any machine.
  • The plugin interface. NPAPI was dropped by every major browser precisely because plugins ran with broad access to the operating system — the background is in what NPAPI was and why browsers dropped it.
  • Silverlight. Microsoft's lifecycle notice confirms Silverlight support ended on October 12, 2021, so the runtime gets no patches either.

There are also practical annoyances: an old browser increasingly fails on the modern web (sites drop support for old TLS versions and web features), automatic updates must be forcibly disabled or the browser will upgrade itself past the cutoff, and many old installers are no longer distributed through official channels — which pushes people toward third-party download sites of unknown trustworthiness.

When people still do it anyway

The honest use case is narrow: a short, contained look at a legacy internal app — for example, reading screens during a migration inventory when no one can find the source code or documentation. Even then, the sensible pattern is an isolated, disposable environment (a virtual machine with no access to anything sensitive), used briefly and then discarded, rather than an old browser sitting on a daily-use workstation. The trade-offs of keeping any of this stack alive long-term are laid out in the risks of keeping a Silverlight app deployed.

The better question

"Which Firefox still runs Silverlight?" usually stands in for a different question: "how do I keep reaching this app?" For that, the durable answers are migration paths — Blazor for web delivery, WPF for desktop — not browser archaeology. An old Firefox build can buy a few minutes of screen-reading time in an emergency; it cannot buy a supported way to keep using the application.

Sources

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