It's been mentioned a bunch in this thread already, but they're doing feature support detection by user-agent string. It's not malicious, it's sloppy.
Here's a quote from the /r/linux thread [1], courtesy of /u/angellus:
> This is certainly a bug. It is not a targeted attacked against Linux users. Report it to Microsoft.
Microsoft is just doing really shitty feature detection using User Agent strings instead of, well actual feature detection. I was able to reproduce the same result by setting my User Agent to Firefox 52 on Windows 98. If I set it to a "more realistic" user agent like IE 7 on Windows XP, it would actually redirect me to a busted page to upgrade my browser instead of Word Online. It appears if Microsoft cannot figure out your User Agent (including your OS as part of it), it gives you a busted experience.
That commenter appears not to have heard of browser detection or UA sniffing, it has nothing to do with feature detection, they're a completely different method (and paradigm).
Nor should browser detection cause Firefox to follow a different switch in the code when on a different OS, not since about version 2 when I think there were bugs making that useful.
Really browser detection was something needed to handle MSIE way back in the day, or possibly other browsers before web fonts.
But this ain't browser detection, definitely not feature detection, it's OS detection - the system responds differently when only the OS in the UA string is altered.
Here's a quote from the /r/linux thread [1], courtesy of /u/angellus:
> This is certainly a bug. It is not a targeted attacked against Linux users. Report it to Microsoft. Microsoft is just doing really shitty feature detection using User Agent strings instead of, well actual feature detection. I was able to reproduce the same result by setting my User Agent to Firefox 52 on Windows 98. If I set it to a "more realistic" user agent like IE 7 on Windows XP, it would actually redirect me to a busted page to upgrade my browser instead of Word Online. It appears if Microsoft cannot figure out your User Agent (including your OS as part of it), it gives you a busted experience.
1 https://www.reddit.com/r/linux/comments/60nj67/office_365_on...