<![CDATA[You can do a GPO to force default applications for computers in a given AD container.
The GPO is in Computer Configuration->Policies->Administrative Templates->Windows Components->File Explorer.
Turn on "Set a default associations configuration file" and point it to the xml file containing your default setting.
To create the XML file the easiest way is to simply set your defaults on a system and then export them.  Use the exported file (delete what you don't want to force) to set the new GPO defaults.  Open an elevated command prompt and run

dism /online /Export-DefaultAppAssociations:"c:\DefaultAppAssociations.xml"

This will make the file you will then point to in your GPO.  Make sure to put it in a location your users can access.
Examples for Chrome and Firefox.
Forcing Google Chrome
<?xml version=”1.0″ encoding=”UTF-8″?>
<DefaultAssociations>
<Association Identifier=”.3gp2″ ProgId=”WMP11.AssocFile.3G2″ ApplicationName=”Windows Media Player” />
<Association Identifier=”.htm” ProgId=”ChromeHTML” ApplicationName=”Google Chrome” />
<Association Identifier=”.html” ProgId=”ChromeHTML” ApplicationName=”Google Chrome” />
<Association Identifier=”.MP2″ ProgId=”WMP11.AssocFile.MP3″ ApplicationName=”Windows Media Player” />
<Association Identifier=”.mpeg” ProgId=”WMP11.AssocFile.mpeg” ApplicationName=”Windows Media Player” />
<Association Identifier=”.oxps” ProgId=”Windows.XPSReachViewer” ApplicationName=”XPS Viewer” />
<Association Identifier=”.tif” ProgId=”PhotoViewer.FileAssoc.Tiff” ApplicationName=”Windows Photo Viewer” />
<Association Identifier=”.tiff” ProgId=”PhotoViewer.FileAssoc.Tiff” ApplicationName=”Windows Photo Viewer” />
<Association Identifier=”.txt” ProgId=”txtfile” ApplicationName=”Notepad” />
<Association Identifier=”.url” ProgId=”IE.AssocFile.URL” ApplicationName=”Internet Browser” />
<Association Identifier=”.website” ProgId=”IE.AssocFile.WEBSITE” ApplicationName=”Internet Explorer” />
<Association Identifier=”.xps” ProgId=”Windows.XPSReachViewer” ApplicationName=”XPS Viewer” />
<Association Identifier=”.htm” ProgId=”ChromeHTML” ApplicationName=”Google Chrome” />
<Association Identifier=”.html” ProgId=”ChromeHTML” ApplicationName=”Google Chrome” />
<Association Identifier=”http” ProgId=”ChromeHTML” ApplicationName=”Google Chrome” />
<Association Identifier=”https” ProgId=”ChromeHTML” ApplicationName=”Google Chrome” />
</DefaultAssociations>
Forcing Mozilla Firefox
<?xml version=”1.0″ encoding=”UTF-8″?>
<DefaultAssociations>
<Association Identifier=”.3gp2″ ProgId=”WMP11.AssocFile.3G2″ ApplicationName=”Windows Media Player” />
<Association Identifier=”.MP2″ ProgId=”WMP11.AssocFile.MP3″ ApplicationName=”Windows Media Player” />
<Association Identifier=”.mpeg” ProgId=”WMP11.AssocFile.mpeg” ApplicationName=”Windows Media Player” />
<Association Identifier=”.oxps” ProgId=”Windows.XPSReachViewer” ApplicationName=”XPS Viewer” />
<Association Identifier=”.tif” ProgId=”PhotoViewer.FileAssoc.Tiff” ApplicationName=”Windows Photo Viewer” />
<Association Identifier=”.tiff” ProgId=”PhotoViewer.FileAssoc.Tiff” ApplicationName=”Windows Photo Viewer” />
<Association Identifier=”.txt” ProgId=”txtfile” ApplicationName=”Notepad” />
<Association Identifier=”.url” ProgId=”IE.AssocFile.URL” ApplicationName=”Internet Browser” />
<Association Identifier=”.website” ProgId=”IE.AssocFile.WEBSITE” ApplicationName=”Internet Explorer” />
<Association Identifier=”.xps” ProgId=”Windows.XPSReachViewer” ApplicationName=”XPS Viewer” />
<Association Identifier=”.htm” ProgId=”FirefoxURL” ApplicationName=”Firefox” />
<Association Identifier=”.html” ProgId=”FirefoxURL” ApplicationName=”Firefox” />
<Association Identifier=”.htm” ProgId=”FirefoxURL” ApplicationName=”Firefox” />
<Association Identifier=”.html” ProgId=”FirefoxURL” ApplicationName=”Firefox” />
<Association Identifier=”http” ProgId=”FirefoxURL” ApplicationName=”Firefox” />
<Association Identifier=”https” ProgId=”FirefoxURL” ApplicationName=”Firefox” />
</DefaultAssociations>]]>