Try [[Clear cookies and site data in Firefox#w_clear-cookies-for-the-current-website|clearing site cookies]] and [[Enhanced Tracking Protection in Firefox for desktop#w_what-to-do-if-a-site-seems-broken|disabling Tracking Protection]] for the affected sites. Does it still happen in [[Diagnose Firefox issues using Troubleshoot Mode|Troubleshoot Mode]]?

HSBC works in troubleshoot mode. nextdoor.co.uk still wont open, get Internal Server Error each time.

For HSBC, try [[Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems#w_disable-all-extensions|disabling your extensions]]. Does the Nextdoor network failure occur before or after the Facebook login popup window appears? Try changing [[Configure DNS over HTTPS protection levels in Firefox|DNS over HTTPS (DoH)]] to "Off" or "Max Protection" and in [[Connection settings in Firefox|Connection Settings]] change "Use system proxy settings" to "No proxy".