User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

1. Go to about:preferences#general
2. Scroll down to Applications
3. Set content type "MSI file" to "Use Windows Installer"
4. Click an MSI file download of your choice, e. g. https://7-zip.org/a/7z1900-x64.msi
5. Confirm Firefox's security warning about executable files

Actual results:

Informational dialog, titled "Windows Installer" and listing all possible command line parameters, is shown.

Reason: msiexec.exe is issued without needed parameter "/i".

Expected results:

Installation of MSI package should begin, e. g. by allowing to specify additional parameters within Firefox's "Select Helper Application" dialog (opened up by clicking "Use other..." at aforementioned step #3).

On a clean profile, there is no "MSI File" entry in the list, and there is no "open with" option when downloading the example you specified, only "Save file" vs. "Cancel".

What other steps are required to reproduce this?

(In reply to :Gijs (he/him) from comment #1)

On a clean profile, there is no "MSI File" entry in the list, and there is no "open with" option when downloading the example you specified, only "Save file" vs. "Cancel".

What other steps are required to reproduce this?

My work machine has such an entry (set to "Save file"), too.
Changing it to "Use other" presents me with the "Select Helper Application" dialog where "Windows installer" is the first and already selected entry - which completes aforementioned steps to reproduce.

Though, I have no idea how this entry was created - neither at home nor at work.

So the fundamental issue of not being able to pass arguments is on file as bug 281903. I'll dupe forward to not lose track of this, though I don't expect someone to work on this in the near future either way.

Looks like 57420 is an even older record of this request. Also bug 321306 for protocol handlers.