I have this exact same issue from today using Win10Pro and FF 88.0 and have followed the same steps you outlined above without success. The Allow button, when it did appear and was clicked, did not respond and mostly I received the message that the (login) webpage was not being directed correctly possibly due to blocked permissions. I added the extension login address as an exception as well as evernote.com in general in Privacy. No change.

Wondering aloud that if this was an Evernote privacy issue, why would only FF be affected? The web clipper works in both Brave and MS Edge.

Hoping for a quick resolution.

Edited to add that I am currently using the Legacy version of Evernote, should that have any impact on this issue. The Web Clipper is the latest version 7.13.8

Same problem I am having. I also cannot access the evernote website thru a search in firefox, however a link in an email works fine in getting me there. The error messages and log files seem to indicate it is related to cookies somehow, but not in a straightforward way.

Everything works fine in the Windows Edge browser. I think Evernote web clipper was just updated, but Firefox was was also updated recently IIRC ( couple days ago?}.

I have the same problem, running on windows 10. No problem on Crome but I cant gain access to the clipper on Firefox on my Mac either. I use it on a daily basis.

According to Evernote support, this issue was resolved on May 17, 2021. https://discussion.evernote.com/forums/topic/136255-fixed-on-51721-suddenly-cant-login-to-the-web-client-from-firefox/