Unlike other images, WebP images do not have colour correction applied in Firefox. This causes visual issues when images using the same colours are used next to each other or overlaid, because only some (but not all) upload.wikimedia.org PNG thumbnails are served as WebP.
I've just uploaded File:BSicon RACONTgq.svg. The colours I've used are F00 and FF0, which should be interpreted as FF0000 and FFFF00, but librsvg colours them differently. Even more confusingly, File:BSicon RAl.svg uses the six-digit colours, but the rendering still uses the incorrect colours, and only in Firefox 57 and not in Tor Browser (Firefox 52 ESR) or Safari 11. This could be a Firefox bug or because of a change in how librsvg works. The below file description pages look identical on Tor Browser and Safari but don't on Firefox 57 (I cleared my cache twice and it persists).
|BSicon RAr (2017)||BSicon AKRZrf (2011)|