Yet again Microsoft knows best.  Released as part of a security update, Microsoft may have quietly installed an extension on your Firefox browser called .NET Framework Assistant.  While it would've been nice to have been asked if you wanted it, it would've been even nicer if it didn't put a huge security hole in your browser!  Their extension allows web sites to quietly install and run anything on your computer without your consent.
Oh, and if you try to remove it (since they've disabled uninstalling for this extension), you could pooch your computer.  Thanks Microsoft.
Source: http://voices.washingtonpost.com/securityfix/2009/05/microsoft_update_quietly_insta.html

If you got stuck with this "feature" that turns Firefox into something about as secure as IE, and want to remove it, here are the instructions:
http://blogs.msdn.com/brada/archive/2009/02/27/uninstalling-the-clickonce-support-for-firefox.aspx
Note this involves tampering with your system registry.

Makers of Firefox have had enough of Microsoft's exploitable and unremovable plugins, and have today issued and update for Firefox browser that disables the troublesome add ins.

What do those two pieces of software do anyway? (other than cause problems)

According to Wikipedia, Windows Presentation Foundation is

Quoted Text

a graphical subsystem for rendering user interfaces in Windows-based applications. WPF was initially released as part of .NET Framework 3.0. Designed to remove dependencies on the aging GDI subsystem, WPF is built on DirectX, which provides hardware acceleration and enables modern UI features like transparency, gradients and transforms. WPF provides a consistent programming model for building applications and provides a clear separation between the user interface and the business logic.

Source:http://en.wikipedia.org/wiki/Windows_Presentation_Foundation

Translation: Makes pretty pictures in your browser

And Microsoft .NET Framework Assistant

Quoted Text

adds ClickOnce support and the ability to report installed .NET versions to the web server.

Source:https://addons.mozilla.org/en-US/firefox/addon/9449

Translation: Tells Microsoft servers what version of .NET you have installed

Both of these are extremely Microsoft-centric, and you'll only ever need them if you're doing Microsofty things.  It's pretty annoying that Microsoft would install them into my non-IE browser without my consent or knowledge.