Since recently (can't remember exactly, perhaps a week) I often get a FOUC in diff views: The page first renders without any CSS (or perhaps only my personal CSS), especially the whole Vector skin styles are missing (two columns, tabs, etc.). These styles are applied only a moment later.
I'm not sure whether this is a bug in MW or in Firefox, and I can't reliably reproduce. I'm putting this here in the hope others can share there own observations, to figure out where exactly the issue is.

For me, the FOUC happens:

• in Firefox
• only when I'm logged in (I tried several times to reproduce in a private window, but everything was fine there)
• most of the time for the first diff view every day (subsequent views usually are fine, even if I hard refresh a page)
• more often for large diffs than for small ones
• only on diff pages, not on normal page views or any other pages I visit