Bugzilla

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36

Steps to reproduce:

This only applies to Windows 10 Anniversary Update or later (i.e. a Windows version where the "PreferSystem32Images" mitigation policy has an affect).

1. Install Firefox release or nightly.
2. Install Skype for Business from the Windows Store.
3. Run Firefox. Go to the address bar, paste https://www.dropbox.com/s/aptwoke9ntvwddm/test.ocsmeet?dl=0 and press enter.  Alternatively, visit this bug page and download the attached test.ocsmeet file.
4. On the web page, click Download > Direct download
5. Firefox will display a window asking you whether to open or save the file.  Select "Open with" and ensure "Skype for Business" is selected in the combo box.  Then click OK.
3. On the "

Actual results:

I am shown the message box:

lync.exe - Entry Point Not Found
The procedure entry point InitializeCriticalSection could not be located in the dynamic link library C:\Program Files (x86)\Microsoft Office\root\Office16\RtmMvrAs.dll. 
OK   

Expected results:

Skype for Business starts.  Note that the test.ocsmeet file is not valid - but you should at least be asked to sign in.