Fellow Firefox/Ubuntu user here. Are you blocking Captchas somehow? This is usually my problem (and specifically very recently with LinkedIn, hence the comment).
I don't think I have changed anything related to captchas, at least not intentionally.
I see a lot of "Request to access cookies or storage on https://www.linkedin.com/ was blocked because of custom cookie permission." in console.
However, my Preferences > Content Blocking is set to "Standard"; no add-ons are enabled; not a private window. I have DNS-over-HTTPS enabled. There are no linkedin-related entries in about:config.
I see a lot of "Request to access cookies or storage on https://www.linkedin.com/ was blocked because of custom cookie permission." in console.
However, my Preferences > Content Blocking is set to "Standard"; no add-ons are enabled; not a private window. I have DNS-over-HTTPS enabled. There are no linkedin-related entries in about:config.