Look at this old Q, maybe that's the answer

Link

I can't see 'Tools' in the about:permissions page, as the referred answer says (only: Store Passwords; Share Location; Use Camera; Use the Microphone; Set Cookies; Open Pop-up Windows; Maintain Offline Storage; Fullscreen)

If I click 'Manage Cookies' for All Sites, Firefox freezes

It was either tools or about:permission. But the real thing is if you have any add-on that prevents cookies look at the last entrance. On about:permission you also have a button (right top corner) forget for this website

You can also look at Websites_report_cookies_are_disabled

I have add-ons that block some cookies, to prevent tracking and other privacy violations, but those had been installed long before this issue emerged.

All other sites, obviously with cookies, work correctly here, and so does Yahoo Mail in another browser.

I guess my other problem, that I can't delete cookies, stems from the same root-issue.

Incidentally, I've tried and found the same problem with Waterfox, the 64 bit variant of Firefox.

Try starting in safe-mode and see if that works. If so I can only suggest that you inactivate that add-on or maybe update it.

In safe-mode I get the same error message: The browser you're using refuses to sign in. (cookies rejected)

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

You can check if DOM Storage is enabled.

• http://kb.mozillazine.org/dom.storage.enabled

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

I couldn't delete this, like the cookies (issue #1 above).

I had un-, and re-install but then, of course, all customization, extensions, etc. were left; so were the issues. Now I cleared everything with Your Uninstaller. Although I had to redo all personalization, it's working at last.