Firefox 115 is the last version that runs on 10.12, 10.13, and 10.14 (also Windows 7 and 8). At this point 115 is 2 years old and GitHub is only tested on bleeding edge browsers, apparently.
So GitHub is usable but there are a number of UI layout issues and searching within a file is sometimes a mess (eg, highlighting the wrong text, rendering text incorrectly, etc. maybe that's true for all browsers. you're better off viewing a file as text in raw mode)
Does that change anything compared to running an old version of Safari that's also extremely slow because of a bug to boot? Modern websites are broken on abandoned browsers anyway, but if you install a less-abandoned browser you'll have a better experience on average.
Firefox doesn't work on Windows 7 anymore but installing Firefox is still a hell of a lot better than sticking to IE.
