I’m hitting it with Linux 64-bit Firefox Nightly 59a1 (2018-01-21). One other person has reported the same with Firefox 57.0.4 (which triggered this report in the first place).
It does the same thing for me here on meta.discourse.org so it’s not specific to our instance, though I’ll admit it might be a browser quirk. However, the fact I encountered the same behaviour around the same time as another user makes me suspicious.
I’ll roll through some more browsers/settings tomorrow; with fresh profiles neither Chromium (63.0.3239.132) nor Firefox (57.0.4) exhibit the same behaviour for me.
Edit:
Little update: it does appear to be a browser setting. Nightly 2018-01-21 and a clean profile doesn’t exhibit the issue, but a -safe-mode start (no extensions) with my normal profile does.
This appears to be a quirk in some profile setting and Firefox Nightly (~2018-01-21/22). Using the same profile in 57.0.4 doesn’t exhibit the same issue.
I can’t see which setting would make this happen so it may well just be a Nightly quirk; the other user who had the issue no longer does with Firefox Nightly, so who knows…
Apologies for the noise…
Edit, someone else just started having the issue with 59b2 (Developer). No idea what’s going on here. I’ll post back here if I can find some sort of pattern to it.
Edit, it’s a Firefox thing. I’ve just seen the same behaviour on GitHub, so “something” is sending the wrong keystrokes to the page.