Earlier quoted context omitted.
If a user agent change makes the software slower for a platform then that's not exactly an accident so somebody must have had the intention to make it run buggy. This has nothing to do with 'making it run well on platforms you care about' but it is 'making it run bad on platforms you don't care about'. It takes more code for the software to act in this way (this is what proves this is intentional and not accidental),…
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…
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.