A mystery: On this page, two tabs in Firefox show slightly different brightnesses for a color image from one tab to another. This is clear if I click one tab, then the other, precisely comparing them (on the same monitor).
If I copy the URL into multiple tabs, some are brighter and some darker. (So it's not that my monitor brightness changed just as I opened a tab.)
But in general, this will be hard for you to reproduce.
These are all ordinary tabs, not Private or Incognito.
The black-and-white image does not vary.
webp files are the same.
Perhaps some sort of A/B testing on CSS? I inspected the CSS and didn't find anything
Firefox 88.0.1 on Macbook OSX 10.15.7