For example:

– Waterfox 56.2.3 on FreeBSD-CURRENT, with a new profile.
Affecting pages such as https://addons.mozilla.org/addon/loremizer/
Not reproducible in Firefox, Chromium, Surf or Web.
Layout in browsers such as Falkon is quite different but (at a glance) not comparably bugged.
Something funky is going on there @grahamperrin but unfortunately I don't think the devs here can help as Waterfox is separate from Firefox 🙁 - on their homepage, it states

I would definitely try recreating this exact issue over on their side, found here.
Thanks,
… unfortunately I don't think the devs here can help …
OK, spun off to the support area at https://redd.it/9jv6lj
As a side note,
… Waterfox is separate from Firefox :slightly_frowning_face: …
That dis-association in the web site footer is, I think, a bit too terse and/or (UPPERCASE) shouty. Maybe interpreted as rage against Mozilla (in which case, I can understand people frowning) but honestly, I don't sense rage. Recently at https://www.reddit.com/comments/9dc129/-/e5hrjje/ the founder wrote:
… assumption that developing Waterfox automatically makes me anti-Mozilla 🤷♂️. Quite the opposite. I have nothing but respect for Mozilla …
FWIW I reckon, that part of the footer is (partly) to discourage users of Waterfox from seeking Waterfox-specific help in Mozilla support areas.
@grahamperrin we merged a fix recently, is it better now? (can you try on addons-dev.allizom.org?)
First load of a page: not fixed, when I reloaded that particular page it appeared to be fixed but after that, all other pages that I tested seem to be not fixed (sorry).
The button briefly 'slides' into place but after a split-second, it reverts to the left.
Screen recording available on request.
Thanks for your attention to this :-)
Screen recording available on request.
I'd be interested in that yep, thanks!
Outdated (updating), unsupported Firefox ESR 52.9.0 on outdated Mac OS X 10.9.5:

Would you still like the screen recording?
Screen recording: 2018-10-17 17:46.mp4.zip
Thanks for your very first contribution @HosseinMarzban! /cc @caitmuenster
Also if you're wondering why I'm showing up in the commits, I helped @HosseinMarzban clean up the commits in his PR as it covered two issues instead of one - he implemented this patch all on his own!
Verified on Firefox ESR 52.9.0 and Waterfox 56.2.5 on Windows 10 and mac OS10.13.6
here's the FF ESR with Win10 - what's now in prod and then the fix

Fix verified with Waterfox 56.2.6 on FreeBSD-CURRENT. Thanks!
Most helpful comment
Fix verified with Waterfox 56.2.6 on FreeBSD-CURRENT. Thanks!