Are third-party cookies enabled? https://support.mozilla.org/en-US/kb/enable-and-disable-cookies-website-preferences

Did this issue persist on safe mode?

Hi Mark Thanks for your reply. The problem persists in Safe Mode, yes. It persists when I uncheck the box 'Tell web sites that I do not want to be tracked.' Also for when third party cookies are enabled.

I'm thinking of doing a complete clean reinstall; by that I mean one in which I delete the Mozilla Firefox directory.

You may have a corrupt cookies.sqlite file.

Type about:support in the address bar and press enter.

Under the page logo on the left side you will see Application Basics. Under this find Profile Folder. To it’s right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the cookies.sqlite file. Then rename or delete it. Restart Firefox.

Hi Fred Thanks for answering, and indeed that solution looked promising. But I'm afraid to say it hasn't helped.

If any of this is any use... I also have a wordpress.com site which I use for testing. I can log on to that with Firefox, no problem.

The wordpress.org site is hosted via Bluehost, using the Thesis theme, by DIY. This situation has baffled very experienced and knowledgeable people in the DIY forums. Posted it on www.wordpress.org/support with 0 replies.

Remove History For One Site Open the History Manager <Control><Shift> H. In the search bar, enter the name of the site. Right click on one of the listings and select Forget About This Site. This should remove all information, including any passwords / settings.

Thanks again for your response. Tried that and regrettably the problem is still there.

You can check the HTTP request/response headers in the Network log.

You can check the Net log and the Network log in the Web Console (Firefox/Tools > Web Developer) to see what cookies are requested to make and which are send.

• https://developer.mozilla.org/Tools/Network_Monitor

You can check if DOM Storage is enabled.

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

Live Http Headers

• https://addons.mozilla.org/firefox/addon/live-http-headers/

Thanks Cor-el. Using about:config I see that dom.storage is 'enabled.' I get this figure. dom.storage.default_quota;5120

For the other two things, I have saved the results on text files viewable here. Link I'm afraid I don't know how to read the information in these log files.

Thanks to everyone who replied. Just like to mention I fixed it, in a rather wholesale way, by un-installing, and then removing the 'Firefox' directory under: C:\Users\YOUR_NAME\AppData\Roaming\Mozilla

...and reinstalling.

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.

Thank you for that. I've taken careful note of it and will use it in the future if this problem reappears.