I am on Linux 64-bit, running Ubuntu 20.04 with Firefox 98. The issue I'm seeing is that all attempts to log in to Dropbox.com result in this message:
Your browser appears to be blocking dropboxcaptcha.com, which is required for authentication. To resolve this issue, please update any anti-virus or ad-blocking software settings to allow connections to dropboxcaptcha.com.
I promise this is not an addon/cookie issue: I have made a clean install of the browser, deleted all of its data in the process, etc., to no effect. There is no "anti-virus or ad-blocking software" running on this machine when I make these attempts.
I can log in fine on Chromium.