UTF-8 added when downloading Word documents from Outlook online

[CanadianUser]

I’ve encountered a very recent issue when downloading attachments via Outlook Web Access (OWA) in Firefox. The downloaded Word documents are being saved with “UTF-8” prefixed to the filename (e.g., UTF-8_example.docx). This doesn’t occur in Chrome or Edge and seems specific to how Firefox handles the Content-Disposition header when served by Outlook Webmail.

It appears to be related to the way filenames are encoded using the filename*= parameter with UTF-8. Firefox may be misinterpreting this or OWA may not be formatting the headers properly.

[siffemcon]

Follow https://bugzilla.mozilla.org/show_bug.cgi?id=1961710.

[dheydrick]

Is Mozilla planning on fixing this bug anytime soon?

[siffemcon]

What is "any time soon"?? The bug was just added two days ago. It will likely take a few weeks for it to land in the release versions.

[dheydrick]

Yes, any time soon. Version 137.0.2 dropped on 4/15/25. I'm betting that I'm not the only customer Mozilla is currently pissing off and potentially losing right now. So, my hope is days rather than weeks.

[siffemcon]

Sorry to disappoint you, but it's actually an Outlook problem, not FF.

https://bugzilla.mozilla.org/show_bug.cgi?id=1961710#c17

[DJJ]

A Microsoft 365 Support Engineer I brought this to the attention of claims that it is a problem for Firefox to fix.  I showed her this thread pointing to your analysis....

[siffemcon]

Yeah, MS broke it so Moz has to fix it. Not an uncommon occurrence.

[fiache]

You know the old adage:

Software: "Its broken! They will fix it in Hardware!"

Hardware: "Its broken! They will fix it in Software!"

[fiache]

BTW, Mahalo for the link above, nice explanation. 

Only curious thing is what is Chrome and Edge doing that avoids this(checked in Edge, don't have Chrome on this machine)?  And why is Mozilla not following suit?

Have a great day.