Suddenly can't log in to certain sites

I would test this as a Firefox-site-compatibility problem rather than a new setting aimed at commerce sites. Since it happens on two computers, first check whether both are on the same Firefox version and same network/DNS/VPN/security software. In Firefox, try one affected site in Troubleshoot Mode, then in a new temporary profile from about:profiles. If the new profile still hangs, open DevTools > Network before logging in and see whether one request is blocked or looping; that gives support something concrete. Also check Enhanced Tracking Protection for the site from the shield icon and try Standard protection just for testing. I would not keep lowering cookie/popup settings globally.

Thanks for your reply. To confirm, both computers are up-to-date with Firefox and are running the same Norton security suite (which is not a new installation for them) and are on a router for the same home internet connection. I assume this fundamentally is a Firefox vs. site compatibility problem, but since it came up after Firefox was updated routinely, it seems to point to a change in Firefox. Maybe it's coincidental that the sites I'm having problems with all are money/commerce related.

I forgot to mention originally that pop-up blocking in Firefox has been turned off throughout this in case that was the problem, and everything is at Standard Tracking Protection, not Enhanced.

So I did the steps you suggested, focusing on the credit union site [Troubleshoot mode, temporary profile, DevTools>Network]. There was a long time spent on "transferring data from cmp.osano.com"; that's a data privacy company. It eventually moved on to what should have been the login screen and spent 5 minutes showing interactions with "rb4.alkami.tech" (a digital banking tool) with occasional pings to the credit union web address. I don't know if that was indicating looping or something else. (After 5 minutes I gave up on it.)

Is anything there useful??

Norton 360 seems to have been blocking secure connections and breaking sites in the last few days according to reports. Would you be able to turn off "Safe Web" feature in Norton settings and try again if anything changes for you? Thanks.

I didn't see the reply from jbr on 6/15. But there has been a Firefox update within the past few days, and now everything seems to be OK (even login access to a Yahoo email account that has been failing for months). I didn't make any changes to Norton 360, so I think something got corrected in Firefox (maybe inadvertently?) in the latest update.

I am experiencing the same issues for a while now. I have been trying to figure it out but by now I have confirmed from 2 support sites/hotlines that this seems to be a firefox compatibility issue of some kind. Anyone from Mozilla reading? I am a firefox user for a long time but this now is becoming problematic as one of the sites is my online banking. calling in several times to resolve it the finite answer was it is an issue with firefox (and only firefox). The advice was to use Chrome, Edge or Safari. Since I am using Firefox because I do not want to use Chrome or Edge for online banking (don't know about Safari but my PC is on Windows...) this is most dissatisfying .. Has anyone an idea or solved the issue in the meantime? BTW another online tool of a client of mine (not commerce/banking related) I am regularily working in, does not show scrollbars anymore (firefox-specific they told me) This makes working online close to impossible. Advice see above...