With the firefox extension enabled, some websites are being rendered before styles have been applied.
Firefox 105.0b9, LastPass for Firefox 4.101.0
This has definitely become more prevalent recently, although I'm not sure exactly when (the last extension update, in early August, seems plausible, though I would have guessed slightly more recent than that).
We're a CMS developer and the LastPass extension is causing us a similar problem.
The prompt to save the password cuts half our editing window. It doesn't do it in every sites. We are not quite sure why.
An hidden images gets added to the DOM and that pushes a bunch of things out on the page. Disabling "Use improved save and fill" seems to suppress the behaviour.
I've been experiencing the "flash of unstyled content" on Firefox within the last year.
I research the problem now and then to see whether I can fix it. Today I found this post, so I tried disabling the LastPass extension. Sites load normally for me when I disable LastPass.
Firefox Version 108.0.1 (64-bit)